# Consumption of Post-Larval Swordfish (Xiphias gladius) by Dolphinfish (Coryphaena hippurus): New Ecological Insights into both Species in the Tyrrhenian Sea

François Poisson

► To cite this version:

François Poisson. Consumption of Post-Larval Swordfish (Xiphias gladius) by Dolphinfish (Coryphaena hippurus): New Ecological Insights into both Species in the Tyrrhenian Sea. Fishes, 2024, 9 (2), 65 (9p.). 10.3390/fishes9020065. [https://doi.org/10.3390/fishes9020065]

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# Consumption of Post-Larval Swordfish (Xiphias gladius) by Dolphinfish (Coryphaena hippurus): New Ecological Insights into both Species in the Tyrrhenian Sea

François Poisson

► To cite this version:

François Poisson. Consumption of Post-Larval Swordfish (Xiphias gladius) by Dolphinfish (Coryphaena hippurus): New Ecological Insights into both Species in the Tyrrhenian Sea. Fishes, 2024, 9 (2), 65 (9p.). 10.3390/fishes9020065. [https://hal.science/hal-04540145v1](https://hal.science/hal-04540145v1)

HAL Id: hal-04540145

Submitted on 28 Jun 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

Technical Note

# Consumption of Post-Larval Swordfish (*Xiphias gladius*) by Dolphinfish (*Coryphaena hippurus*): New Ecological Insights into Both Species in the Tyrrhenian Sea

François Poisson

MARBEC, Univ Montpellier, CNRS, Ifremer, IRD, 34203 Sète, France; francois.poisson@ifremer.fr

**Abstract:** Knowledge of post-larval swordfish (*Xiphias gladius*) ecology in the world's oceans is incomplete as they are rarely found in ichthyoplankton samplings or commercial catches and individuals are difficult to observe in the marine ecosystem. Analyses of stomach contents of apex predators can provide otherwise unobtainable ecological insights. Two well-preserved bills of post-larval swordfish were identified among the partially digested stomach contents of an adult male dolphinfish (*Coryphaena hippurus*) caught in the northern Tyrrhenian Sea. The lower jaw to fork lengths of the two predated swordfish were estimated at, respectively, 18 and 22 cm, corresponding to 81- and 99-day-old swordfish hatched in the 2022 spawning season. This analysis and recent information on the reproductive dynamics of swordfish in the same area reported in the literature shed light not only on the early stages of swordfish growth but also on dolphinfish ecology in the Tyrrhenian Sea. The two species share the same habitat and are both caught with commercial pelagic surface longlines. Environmental shifts associated with climate change could unpredictably modify the reproduction dynamics, growth, recruitment and distribution of both species. This knowledge is important for the sustainable management of regional fisheries.

**Keywords:** dolphinfish; Mediterranean Sea; post larvae; stomach contents; swordfish; Tyrrhenian Sea

**Key Contribution:** Information on the early stages of development of the Mediterranean swordfish is currently limited as synoptic observations in marine ecosystems are difficult. Two well-preserved bills of post-larval swordfish were identified in the contents of an adult male dolphinfish (*Coryphaena hippurus*) caught in the northern Tyrrhenian Sea. This is the first evidence for consumption of young-of-the-year swordfish by dolphinfish in the Mediterranean. The studied area can be identified as nursery ground for swordfish. From the point of view of conservation, it is vital to determine, using routine and systematic sampling of top-predator-fish stomachs, if post larvae swordfish occur more frequently. This indirect method could enable us to infer additional information about pelagic communities.

## 1. Introduction

The dolphinfish (*Coryphaena hippurus*) and the broadbill swordfish (*Xiphias gladius*) are two large pelagic and highly migratory apex predators which share the same habitat in the western Mediterranean and are caught by surface longline vessels [1–3]. The swordfish is a high-market-value pelagic fish that is heavily fished in the Mediterranean Sea by at least 27 countries using different types of fishing gear [4,5]. Female swordfish are multiple spawners with asynchronous oocyte development, able to spawn every 2.3 to 3 days during the spawning period [6–8]. Based on the distribution of larvae and eggs, the presumed spawning grounds match the 23 to 26 °C surface isotherms of the summer season in temperate areas [9]; although in tropical waters, spawning can take place almost all year round. Swordfish swimming in pairs and displaying courtship behaviours are also mainly observed at temperatures above 24 °C [10].

<figure><img src="image_3.png" /></figure>

**check for
updates**

**Citation:** Poisson, F. Consumption of Post-Larval Swordfish (*Xiphias gladius*) by Dolphinfish (*Coryphaena hippurus*): New Ecological Insights into Both Species in the Tyrrhenian Sea. *Fishes* **2024**, *9*, 65. https://doi.org/10.3390/fishes9020065

Academic Editor: S.S.S. Sarma

Received: 29 December 2023

Revised: 22 January 2024

Accepted: 2 February 2024

Published: 6 February 2024

<figure><img src="image_4.png" /></figure>

**Copyright:** © 2024 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).
<figure><img src="image_3.png" /></figure>

Technical Note

# Consumption of Post-Larval Swordfish (*Xiphias gladius*) by Dolphinfish (*Coryphaena hippurus*): New Ecological Insights into Both Species in the Tyrrhenian Sea

François Poisson

<figure><img src="image_4.png" /></figure>

MARBEC, Univ Montpellier, CNRS, Ifremer, IRD, 34203 Sète, France; francois.poisson@ifremer.fr

**Abstract:** Knowledge of post-larval swordfish (*Xiphias gladius*) ecology in the world's oceans is incomplete as they are rarely found in ichthyoplankton samplings or commercial catches and individuals are difficult to observe in the marine ecosystem. Analyses of stomach contents of apex predators can provide otherwise unobtainable ecological insights. Two well-preserved bills of post-larval swordfish were identified among the partially digested stomach contents of an adult male dolphinfish (*Coryphaena hippurus*) caught in the northern Tyrrhenian Sea. The lower jaw to fork lengths of the two predated swordfish were estimated at, respectively, 18 and 22 cm, corresponding to 81- and 99-day-old swordfish hatched in the 2022 spawning season. This analysis and recent information on the reproductive dynamics of swordfish in the same area reported in the literature shed light not only on the early stages of swordfish growth but also on dolphinfish ecology in the Tyrrhenian Sea. The two species share the same habitat and are both caught with commercial pelagic surface longlines. Environmental shifts associated with climate change could unpredictably modify the reproduction dynamics, growth, recruitment and distribution of both species. This knowledge is important for the sustainable management of regional fisheries.

**Keywords:** dolphinfish; Mediterranean Sea; post larvae; stomach contents; swordfish; Tyrrhenian Sea

**Key Contribution:** Information on the early stages of development of the Mediterranean swordfish is currently limited as synoptic observations in marine ecosystems are difficult. Two well-preserved bills of post-larval swordfish were identified in the contents of an adult male dolphinfish (*Coryphaena hippurus*) caught in the northern Tyrrhenian Sea. This is the first evidence for consumption of young-of-the-year swordfish by dolphinfish in the Mediterranean. The studied area can be identified as nursery ground for swordfish. From the point of view of conservation, it is vital to determine, using routine and systematic sampling of top-predator-fish stomachs, if post larvae swordfish occur more frequently. This indirect method could enable us to infer additional information about pelagic communities.

## 1. Introduction

The dolphinfish (*Coryphaena hippurus*) and the broadbill swordfish (*Xiphias gladius*) are two large pelagic and highly migratory apex predators which share the same habitat in the western Mediterranean and are caught by surface longline vessels [1–3]. The swordfish is a high-market-value pelagic fish that is heavily fished in the Mediterranean Sea by at least 27 countries using different types of fishing gear [4,5]. Female swordfish are multiple spawners with asynchronous oocyte development, able to spawn every 2.3 to 3 days during the spawning period [6–8]. Based on the distribution of larvae and eggs, the presumed spawning grounds match the 23 to 26 °C surface isotherms of the summer season in temperate areas [9]; although in tropical waters, spawning can take place almost all year round. Swordfish swimming in pairs and displaying courtship behaviours are also mainly observed at temperatures above 24 °C [10].

<figure><img src="image_5.png" /></figure>

check for
updates

**Citation:** Poisson, F. Consumption of Post-Larval Swordfish (*Xiphias gladius*) by Dolphinfish (*Coryphaena hippurus*): New Ecological Insights into Both Species in the Tyrrhenian Sea. *Fishes* **2024**, *9*, 65. https://doi.org/10.3390/fishes9020065

Academic Editor: S.S.S. Sarma

Received: 29 December 2023

Revised: 22 January 2024

Accepted: 2 February 2024

Published: 6 February 2024

<figure><img src="image_6.png" /></figure>

**Copyright:** © 2024 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).
A considerable amount of the literature is devoted to swordfish reproduction in the Mediterranean Sea [11]. The Straits of Messina and the southern Tyrrhenian Sea have been identified as the main swordfish spawning grounds, based on observations of eggs, larvae and mature females [9,12,13]. In the Ligurian Sea, that is part of the north-western Mediterranean, spawning takes place between June and September with a peak in July [14]. It is likely that the potential breeding area in the Mediterranean basin is relatively larger than the breeding area in the Atlantic [15,16]. New spawning areas for this species have been reported in the Levantine Basin [17], and other similar areas may exist in the Ionian Sea [4]. Swordfish eggs and larvae are only abundant in July [18]. Recently observed seasonal variations in sex ratio, as well as in the occurrence of females with running eggs, confirm that spawning occurs off the island of Corsica [19]. Swordfish eggs and the earliest development stages of the fish have been described in detail since the 20th century [20] based on individuals sampled in the wild [12,13,21–25] or on eggs hatched in a laboratory [13,25].

Despite the high commercial importance of this species, knowledge of the post-larval
swordfish ecology in the ocean is still incomplete [20,26]. Swordfish larval development is
complex [13,27,28]. Larvae are generally associated with sea surface temperatures (SST)
between 24 °C and 29 °C. Swordfish retain larval characters until their standard length, i.e.,
from the tip of the snout to the end of the middle caudal fin rays (SL) is at least 188 mm [21].
Knowledge of the ecology of post-larval swordfish in the ocean is incomplete due to the
difficulty involved in observing these individuals in the marine ecosystem, as they are
rarely present in ichthyoplankton splittings and in catches.

Information on the spatial distribution of all swordfish life stages, from eggs to pre-
adults, and on how environmental factors affect reproduction, spatial distribution, growth
rate and recruitment of swordfish in the Mediterranean is lacking.

The swordfish is a fast-growing species, particularly in its first year. Juveniles entering the recruitment phase are caught near the coast in the last three months of the year. It is consequently assumed that coastal waters serve as a nursery habitat [29–31]. Positive SST anomalies could favour recruitment [32], while lower temperatures could delay the spawning season and slow down the growth rate of juveniles [33].

Swordfish management in the Mediterranean is under the control of the International
Commission for the Conservation of Atlantic Tunas (ICCAT). As the high percentage of
juveniles caught could have a negative impact on the spawning biomass per recruit; a
closure period for longliners has been implemented. The most recent assessment in 2020
estimated biomass to be at its lowest level since 1950 [5]. As a result, a 15-year recovery plan
was implemented [34]. It is prohibited to have on board, on land, or to transport, store, and
sell swordfish measuring less than 100 cm LJFL (Lower Jaw to Fork Length) even though
at-haulback mortality is thought to be as high as in the Atlantic (ranging between 78% and
88%), and hence the same for the post-release mortality [5]. Countries are encouraged to
find ways to reduce the capture of undersized swordfish [29]. The dolphinfish, a large
tropical/subtropical migratory species, is an opportunistic predator which forages on
epipelagic prey and is known to be associated with floating objects (FOBs) [35]. Acoustic
telemetry data have shown that when associated with FOBs, dolphinfish can spend more
than 95% of their time in the first ten meters below the sea surface [36], but that when not
associated with FOBs and undertaking daily vertical migrations, they swim down to a
depth of 160 m [37].

In the Mediterranean Sea, dolphinfish are targeted by the artisanal fisheries that use
anchored fish aggregating devices (FADs) as well as by the recreational fisheries [38,39].
Commercial captures have increased steadily over time, and it is assumed that climate
conditions in March and April affect the catch rates of dolphinfish longline fisheries in
September and October [2].

When early stages are cryptic, which is the case of swordfish post-larval stages, stom-
ach contents analyses of apex predators can provide otherwise unobtainable ecological
information. A rarely reported predation event on two swordfish larvae by a dolphinfish
A considerable amount of the literature is devoted to swordfish reproduction in the Mediterranean Sea [11]. The Straits of Messina and the southern Tyrrhenian Sea have been identified as the main swordfish spawning grounds, based on observations of eggs, larvae and mature females [9,12,13]. In the Ligurian Sea, that is part of the north-western Mediterranean, spawning takes place between June and September with a peak in July [14]. It is likely that the potential breeding area in the Mediterranean basin is relatively larger than the breeding area in the Atlantic [15,16]. New spawning areas for this species have been reported in the Levantine Basin [17], and other similar areas may exist in the Ionian Sea [4]. Swordfish eggs and larvae are only abundant in July [18]. Recently observed seasonal variations in sex ratio, as well as in the occurrence of females with running eggs, confirm that spawning occurs off the island of Corsica [19]. Swordfish eggs and the earliest development stages of the fish have been described in detail since the 20th century [20] based on individuals sampled in the wild [12,13,21–25] or on eggs hatched in a laboratory [13,25].

Despite the high commercial importance of this species, knowledge of the post-larval
swordfish ecology in the ocean is still incomplete [20,26]. Swordfish larval development is
complex [13,27,28]. Larvae are generally associated with sea surface temperatures (SST)
between 24 °C and 29 °C. Swordfish retain larval characters until their standard length, i.e.,
from the tip of the snout to the end of the middle caudal fin rays (SL) is at least 188 mm [21].
Knowledge of the ecology of post-larval swordfish in the ocean is incomplete due to the
difficulty involved in observing these individuals in the marine ecosystem, as they are
rarely present in ichthyoplankton splittings and in catches.

Information on the spatial distribution of all swordfish life stages, from eggs to pre-
adults, and on how environmental factors affect reproduction, spatial distribution, growth
rate and recruitment of swordfish in the Mediterranean is lacking.

The swordfish is a fast-growing species, particularly in its first year. Juveniles entering the recruitment phase are caught near the coast in the last three months of the year. It is consequently assumed that coastal waters serve as a nursery habitat [29–31]. Positive SST anomalies could favour recruitment [32], while lower temperatures could delay the spawning season and slow down the growth rate of juveniles [33].

Swordfish management in the Mediterranean is under the control of the International
Commission for the Conservation of Atlantic Tunas (ICCAT). As the high percentage of
juveniles caught could have a negative impact on the spawning biomass per recruit; a
closure period for longliners has been implemented. The most recent assessment in 2020
estimated biomass to be at its lowest level since 1950 [5]. As a result, a 15-year recovery plan
was implemented [34]. It is prohibited to have on board, on land, or to transport, store, and
sell swordfish measuring less than 100 cm LJFL (Lower Jaw to Fork Length) even though
at-haulback mortality is thought to be as high as in the Atlantic (ranging between 78% and
88%), and hence the same for the post-release mortality [5]. Countries are encouraged to
find ways to reduce the capture of undersized swordfish [29]. The dolphinfish, a large
tropical/subtropical migratory species, is an opportunistic predator which forages on
epipelagic prey and is known to be associated with floating objects (FOBs) [35]. Acoustic
telemetry data have shown that when associated with FOBs, dolphinfish can spend more
than 95% of their time in the first ten meters below the sea surface [36], but that when not
associated with FOBs and undertaking daily vertical migrations, they swim down to a
depth of 160 m [37].

In the Mediterranean Sea, dolphinfish are targeted by the artisanal fisheries that use
anchored fish aggregating devices (FADs) as well as by the recreational fisheries [38,39].
Commercial captures have increased steadily over time, and it is assumed that climate
conditions in March and April affect the catch rates of dolphinfish longline fisheries in
September and October [2].

When early stages are cryptic, which is the case of swordfish post-larval stages, stom-
ach contents analyses of apex predators can provide otherwise unobtainable ecological
information. A rarely reported predation event on two swordfish larvae by a dolphinfish
in the Tyrrhenian Sea thus provides new insights into the ecology of both species in the area. The abnormally high SSTs recorded in 2022 in the area and the ecologically negative implications of dolphinfish predation are discussed.

**2. Materials and Methods**

**2.1. Sampling**

Gonad samplings were conducted in the vicinity of the island of Corsica, in collaboration with domestic longline swordfish fisheries, from 2019 to 2021 in the framework of the GEN&REC project (European Maritime and Fisheries Fund-EMFF). The aim of the research is to fill the knowledge gap concerning swordfish reproduction biology in the north Tyrrhenian Sea and is in line with research conducted by (ICCAT) [40]. Five females with running eggs were caught between 2019 and 2021 [19], and were measured and sexed during sampling at sea.

On the 23 September 2022, a ripe male dolphinfish with a 97 cm fork length (FL) was caught with a drifting longline baited with *Illex* sp. (13–18 cm mantle length) at ca. 41°49'684'' N; 9°29'562'' E (Figure 1). It was caught alive at 14:10 UTC and rapidly gutted. The stomach of this specimen was opened on the deck. A visual inspection of the fragments of the digested fishes revealed two well-preserved pointed, flat rostrums identified as swordfish bills. The hard parts were isolated and preserved in 90% ethanol for further examination together with the other tissues collected.

**2.2. Estimated Specimen Size, Age and Spawning Time**

Age was estimated based on the results of a swordfish growth study conducted in the eastern Mediterranean [30] and the spawning times back-calculated integrating the swordfish egg incubation period proposed by Yasuda et al. [13].

<figure><img src="image_5.png" /><figcaption>**Figure 1.** *Cont.*</figcaption></figure>
in the Tyrrhenian Sea thus provides new insights into the ecology of both species in the area. The abnormally high SSTs recorded in 2022 in the area and the ecologically negative implications of dolphinfish predation are discussed.

**2. Materials and Methods**

**2.1. Sampling**

Gonad samplings were conducted in the vicinity of the island of Corsica, in collaboration with domestic longline swordfish fisheries, from 2019 to 2021 in the framework of the GEN&REC project (European Maritime and Fisheries Fund-EMFF). The aim of the research is to fill the knowledge gap concerning swordfish reproduction biology in the north Tyrrhenian Sea and is in line with research conducted by (ICCAT) [40]. Five females with running eggs were caught between 2019 and 2021 [19], and were measured and sexed during sampling at sea.

On the 23 September 2022, a ripe male dolphinfish with a 97 cm fork length (FL) was caught with a drifting longline baited with *Illex* sp. (13–18 cm mantle length) at ca. 41°49'684'' N; 9°29'562'' E (Figure 1). It was caught alive at 14:10 UTC and rapidly gutted. The stomach of this specimen was opened on the deck. A visual inspection of the fragments of the digested fishes revealed two well-preserved pointed, flat rostrums identified as swordfish bills. The hard parts were isolated and preserved in 90% ethanol for further examination together with the other tissues collected.

**2.2. Estimated Specimen Size, Age and Spawning Time**

Age was estimated based on the results of a swordfish growth study conducted in the eastern Mediterranean [30] and the spawning times back-calculated integrating the swordfish egg incubation period proposed by Yasuda et al. [13].

<figure><img src="image_7.png" /><figcaption>**Figure 1.** *Cont.*</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 1.** (a) Swordfish bills collected from the stomach of a dolphinfish in the Tyrrhenian Sea in September 2022 (scale: 17 cm). (b) Remains of two swordfish bills and body fragments taken from the stomach of a dolphinfish caught in the Tyrrhenian Sea in September 2022 (scale: 10 cm). (c) Photo of a juvenile swordfish with a LJFL of 23 cm caught in the Aegean Sea (scale: 10 cm). Source: Dr Nota Peristeraki at the Hellenic Centre for Marine Research (HCMR).</figcaption></figure>
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 1.** (a) Swordfish bills collected from the stomach of a dolphinfish in the Tyrrhenian Sea in September 2022 (scale: 17 cm). (b) Remains of two swordfish bills and body fragments taken from the stomach of a dolphinfish caught in the Tyrrhenian Sea in September 2022 (scale: 10 cm). (c) Photo of a juvenile swordfish with a LJFL of 23 cm caught in the Aegean Sea (scale: 10 cm). Source: Dr Nota Peristeraki at the Hellenic Centre for Marine Research (HCMR).</figcaption></figure>
## 3. Results

### Identification, Size and Hatching Day

The stomach contents were identified visually to the lowest taxonomic level. The contents mainly consisted of small fishes digested to varying degrees, plus two larger specimens. The identification was straightforward, with no confusion possible because the shape of the swordfish bill, a pronounced extension of the upper jaw, is unique [41] (Figure 1a,b), while the six billfishes that occur in the Mediterranean are the Mediterranean spearfish (*Tetrapturus belone*), an endemic species, the white marlin (*Tetrapturus albidus*), the Atlantic sailfish (*Istiophorus albicans*), the Atlantic white marlin (*Kajikia albida*), the round-scale spearfish (*Tetrapturus georgii*) and the sailfish (*Istiophorus platypterus*), a lessepsian species [23,24,42] which all have a bill with a round cross section.

Back in the laboratory, the body fragments, tentatively identified as juvenile swordfish were assembled, and their LJFL estimated at, respectively, 18 and 22 cm.

Based on a growth rate of 2.3 mm day⁻¹, the two sizes corresponded to 81- and 99-day-old swordfish, theoretically hatched on, respectively, 16 June and 5 July 2022.

The shape of the two specimens observed resembles the drawing published by Fowler (1924) and the picture of a juvenile with a LJFL 23 cm caught in the Aegean Sea (Greece) (Figure 1c).

## 4. Discussion

The location of spawning swordfish and the analysis of the stomach contents of dolphinfish provided valuable information concerning the spawning grounds, the geographic distribution and spatial aggregation patterns of juvenile swordfish and revealed particular oceanographic features that occurred in 2022.

### 4.1. A Rarely Reported Event

The size of male dolphinfish at first maturity in the western Mediterranean was estimated to be 61.8 cm, and the mean trophic level for large individuals was estimated to be 4.5 ± 0.70 [43,44]. The dolphinfish is an opportunistic daytime predator that can use different foraging strategies depending on the prey availability [35].

A review of the literature on dolphinfish diet showed that both neritic and demersal finfish were among the stomach contents in all of the studies but that dolphinfish also consume crabs and epipelagic cephalopods. Global records of species identified in dolphinfish stomachs have previously not included young swordfish. Young juvenile swordfish were found in the stomach of 10 other predators including billfish, tuna and blue shark. Only one case of a 19.21 cm long swordfish found in the stomach of a dolphinfish captured on the trolling lines has been reported [45]. In the present study, the mature male observed was able to swallow two juvenile swordfish.

It is of course difficult to know whether the fish were alive or dead when they were consumed. However, dolphinfish feed on the surface and are associated with FOBs. It is also difficult to explain how these two individuals could have died almost at the same time (as they were found in a similar stage of decomposition) apart from due to an attack by a predator. It is, therefore, more likely that these are two cases of predation rather than consumption of dead animals.

In these circumstances, this event reflects the ability of adult dolphinfish to catch free-swimming prey that are as long as the bait used by swordfish longliners. The presence of juvenile swordfish in the dolphin's stomach leads to the hypothesis that, at this early stage, swordfish that are more common in epipelagic waters are associated with FOBs.

From the point of view of conservation, it is vital to determine if post-larval swordfish occur more frequently than previously assumed. This can be achieved using systematic sampling of the stomachs of top predator fish. This indirect would enable additional information to be inferred concerning pelagic communities [46].
## 3. Results

### Identification, Size and Hatching Day

The stomach contents were identified visually to the lowest taxonomic level. The contents mainly consisted of small fishes digested to varying degrees, plus two larger specimens. The identification was straightforward, with no confusion possible because the shape of the swordfish bill, a pronounced extension of the upper jaw, is unique [41] (Figure 1a,b), while the six billfishes that occur in the Mediterranean are the Mediterranean spearfish (*Tetrapturus belone*), an endemic species, the white marlin (*Tetrapturus albidus*), the Atlantic sailfish (*Istiophorus albicans*), the Atlantic white marlin (*Kajikia albida*), the round-scale spearfish (*Tetrapturus georgii*) and the sailfish (*Istiophorus platypterus*), a lessepsian species [23,24,42] which all have a bill with a round cross section.

Back in the laboratory, the body fragments, tentatively identified as juvenile swordfish were assembled, and their LJFL estimated at, respectively, 18 and 22 cm.

Based on a growth rate of 2.3 mm day⁻¹, the two sizes corresponded to 81- and 99-day-old swordfish, theoretically hatched on, respectively, 16 June and 5 July 2022.

The shape of the two specimens observed resembles the drawing published by Fowler (1924) and the picture of a juvenile with a LJFL 23 cm caught in the Aegean Sea (Greece) (Figure 1c).

## 4. Discussion

The location of spawning swordfish and the analysis of the stomach contents of dolphinfish provided valuable information concerning the spawning grounds, the geographic distribution and spatial aggregation patterns of juvenile swordfish and revealed particular oceanographic features that occurred in 2022.

### 4.1. A Rarely Reported Event

The size of male dolphinfish at first maturity in the western Mediterranean was estimated to be 61.8 cm, and the mean trophic level for large individuals was estimated to be 4.5 ± 0.70 [43,44]. The dolphinfish is an opportunistic daytime predator that can use different foraging strategies depending on the prey availability [35].

A review of the literature on dolphinfish diet showed that both neritic and demersal finfish were among the stomach contents in all of the studies but that dolphinfish also consume crabs and epipelagic cephalopods. Global records of species identified in dolphinfish stomachs have previously not included young swordfish. Young juvenile swordfish were found in the stomach of 10 other predators including billfish, tuna and blue shark. Only one case of a 19.21 cm long swordfish found in the stomach of a dolphinfish captured on the trolling lines has been reported [45]. In the present study, the mature male observed was able to swallow two juvenile swordfish.

It is of course difficult to know whether the fish were alive or dead when they were consumed. However, dolphinfish feed on the surface and are associated with FOBs. It is also difficult to explain how these two individuals could have died almost at the same time (as they were found in a similar stage of decomposition) apart from due to an attack by a predator. It is, therefore, more likely that these are two cases of predation rather than consumption of dead animals.

In these circumstances, this event reflects the ability of adult dolphinfish to catch free-swimming prey that are as long as the bait used by swordfish longliners. The presence of juvenile swordfish in the dolphin's stomach leads to the hypothesis that, at this early stage, swordfish that are more common in epipelagic waters are associated with FOBs.

From the point of view of conservation, it is vital to determine if post-larval swordfish occur more frequently than previously assumed. This can be achieved using systematic sampling of the stomachs of top predator fish. This indirect would enable additional information to be inferred concerning pelagic communities [46].
## 4.2. Spawning and Nursery Grounds

The migratory nature of swordfish allows them to reach the favourable environmental spawning conditions needed to guarantee successful reproduction. Temperature is a crucial parameter in the distribution of swordfish in all the world's oceans [47]. In the present study, females ready to spawn were captured in three areas with contrasting oceanographic features and water mass circulation (currents and eddies) [19] where sea surface temperatures were above 24 °C, which could explain the short time lag before the beginning of the spawning season and its duration. Above this temperature threshold, growth rates are assumed to be high, consequently reducing the duration of the critical period of high mortality of larval and juvenile stages [48,49]. Poisson et al. [8] demonstrated that a simultaneous decrease in SST and an increase in the prevalence of females is an indicator of the end of the spawning season.

Reproduction in the Ligurian Sea is seasonal, with a peak in July–August and a tail in September, and all stages from post larva to advanced ages have been found there [50–52]. In the vicinity of the island of Corsica, the spawning season has been inferred from the trend of the gonadal index, from histological studies of the gonads and observation of females with running eggs [19]. Spawning occurs in June and July off the coast on both sides of Corsica. The two swordfish found in the dolphinfish stomach were estimated to be hatched in the 2022 spawning season. Optimum temperatures and food availability influence the growth rate of fish larvae [49,53], and it is likely that both trophic and oceanographic conditions favour the development and growth of swordfish in their early life stages. Coastal waters off both sides of Corsica have been identified as nursery grounds for swordfish. The association with FOBs could facilitates the dispersal of juveniles along the coast. It can be assumed that young-of-the-year swordfish remain in the spawning grounds during the first year of their life. Yabe et al. [45] reported that swordfish can reach 50–60 cm in their first year, and then grow between 25 and 38 cm per year.

The smaller swordfish which was less than 90 cm long must have hatched the previous year. There is also evidence that the young of the year prefer coastal waters particularly during the winter months [47].

## 4.3. Environmental Factors and Recruitment

Peristeraki et al. [33] demonstrated that successful recruitment is closely positively correlated with high ambient water temperatures, and that lower temperatures in particular can delay the spawning period and reduce the growth rate of juveniles. The year 1998, qualified as a warm year, was identified as an "exceptional" year for the recruitment of young individuals [32]. Due to the high SSTs recorded over an extended period, one would expect 2022 to be a good recruitment year for swordfish, as both the period of reproduction and its length could have been affected by changes in the SST.

Obtaining this information is critical because predation can affect recruitment. Spawning could have taken place earlier in the season and continued over an extended period.

Scenarios produced by CERES (Climate Change and European Fisheries and Aquaculture, H2020, EU 678193; https://ceresproject.eu/case-studies/ last accessed on 29 May 2023) show that the dolphinfish is a potential "winner" of climate change in the Mediterranean Sea, as it will affect both the spawning season and location together with the growth rate of juveniles. Dolphinfish are predicted to be more frequent throughout the year, and their size and total catches may vary in the near future [54].

## 5. Conclusions

Information on the early stages of development of the Mediterranean swordfish is currently limited as synoptic observations in marine ecosystems are difficult. The unusual and auspicious observation reported here describes a rare event involving two charismatic predators. This is the first evidence for consumption of young-of-the-year swordfish by dolphinfish in the Mediterranean thereby shedding light on swordfish predation by another apex predator species. The impact that predators might have on swordfish stocks
## 4.2. Spawning and Nursery Grounds

The migratory nature of swordfish allows them to reach the favourable environmental spawning conditions needed to guarantee successful reproduction. Temperature is a crucial parameter in the distribution of swordfish in all the world's oceans [47]. In the present study, females ready to spawn were captured in three areas with contrasting oceanographic features and water mass circulation (currents and eddies) [19] where sea surface temperatures were above 24 °C, which could explain the short time lag before the beginning of the spawning season and its duration. Above this temperature threshold, growth rates are assumed to be high, consequently reducing the duration of the critical period of high mortality of larval and juvenile stages [48,49]. Poisson et al. [8] demonstrated that a simultaneous decrease in SST and an increase in the prevalence of females is an indicator of the end of the spawning season.

Reproduction in the Ligurian Sea is seasonal, with a peak in July–August and a tail in September, and all stages from post larva to advanced ages have been found there [50–52]. In the vicinity of the island of Corsica, the spawning season has been inferred from the trend of the gonadal index, from histological studies of the gonads and observation of females with running eggs [19]. Spawning occurs in June and July off the coast on both sides of Corsica. The two swordfish found in the dolphinfish stomach were estimated to be hatched in the 2022 spawning season. Optimum temperatures and food availability influence the growth rate of fish larvae [49,53], and it is likely that both trophic and oceanographic conditions favour the development and growth of swordfish in their early life stages. Coastal waters off both sides of Corsica have been identified as nursery grounds for swordfish. The association with FOBs could facilitates the dispersal of juveniles along the coast. It can be assumed that young-of-the-year swordfish remain in the spawning grounds during the first year of their life. Yabe et al. [45] reported that swordfish can reach 50–60 cm in their first year, and then grow between 25 and 38 cm per year.

The smaller swordfish which was less than 90 cm long must have hatched the previous year. There is also evidence that the young of the year prefer coastal waters particularly during the winter months [47].

## 4.3. Environmental Factors and Recruitment

Peristeraki et al. [33] demonstrated that successful recruitment is closely positively correlated with high ambient water temperatures, and that lower temperatures in particular can delay the spawning period and reduce the growth rate of juveniles. The year 1998, qualified as a warm year, was identified as an "exceptional" year for the recruitment of young individuals [32]. Due to the high SSTs recorded over an extended period, one would expect 2022 to be a good recruitment year for swordfish, as both the period of reproduction and its length could have been affected by changes in the SST.

Obtaining this information is critical because predation can affect recruitment. Spawning could have taken place earlier in the season and continued over an extended period.

Scenarios produced by CERES (Climate Change and European Fisheries and Aquaculture, H2020, EU 678193; https://ceresproject.eu/case-studies/ last accessed on 29 May 2023) show that the dolphinfish is a potential "winner" of climate change in the Mediterranean Sea, as it will affect both the spawning season and location together with the growth rate of juveniles. Dolphinfish are predicted to be more frequent throughout the year, and their size and total catches may vary in the near future [54].

## 5. Conclusions

Information on the early stages of development of the Mediterranean swordfish is currently limited as synoptic observations in marine ecosystems are difficult. The unusual and auspicious observation reported here describes a rare event involving two charismatic predators. This is the first evidence for consumption of young-of-the-year swordfish by dolphinfish in the Mediterranean thereby shedding light on swordfish predation by another apex predator species. The impact that predators might have on swordfish stocks
is unknown, and there is a need for more diet studies of large predatory fish. The spatial distribution and connectivity between swordfish spawning and putative nursery grounds during swordfish ontogeny also requires further investigation. Environmental shifts associated with climate change could unpredictably modify the reproduction dynamics, growth, recruitment and distribution of both species. This knowledge is important for the sustainable management of regional fisheries.

**Funding:** The Gen&Rec project ("Distribution and behaviour of swordfish spawners and juveniles in the vicinity of Corsica") is an IFREMER project funded by the European Union (European Maritime and Fisheries Fund—EMFF), grant number 18/2216415 and by the fishing industry (France Filière Pêche—FFP) grant number 20/1001220 conducted in close collaboration with French swordfish longline fishers with four partners (IRD, Stella Mare (Université de Corse Pasquale Paoli), CEFE and the fisheries committee of Corsica (CRPMEM Corsica).

**Institutional Review Board Statement:** No ethical parameters required as the gutted Dolphinfish (a marketable fish) used for the study was provided by skipper of the fishing boat.

**Data Availability Statement:** All data generated or analysed during this study are included in this published article.

**Acknowledgments:** All the fishers are acknowledged for their active participation in this project, particularly Jean-Toussaint Lucchini and his sons, owners of the vessel "Corail" which caught the dolphinfish concerned. I would like to thank the three reviewers and the Associate Editor for their comments that improved this manuscript.

**Conflicts of Interest:** The author declares no conflict of interest.

## References

1. Garibaldi, F. By-catch in the mesoplastic swordfish longline fishery in the ligurian sea (Western Mediterranean). *Collect. Vol. Sci. Pap. ICCAT* **2015**, *71*, 1495-1498.

2. Báez, J.C.; Camiñas, J.A.; Hernández, P.; Vasconcellos, M.; García-Barcelona, S.; Macías, D. North Atlantic Oscillation affects dolphinfish catch and bycatch in the Western Mediterranean Sea. *Reg. Stud. Mar. Sci.* **2020**, *36*, 101303. [CrossRef]

3. Macías, D.; Báez, J.C.; García-Barcelona, S.; Ortiz de Urbina, J.M. Dolphinfish Bycatch in Spanish Mediterranean Large Pelagic Longline Fisheries, 2000-2010. *Sci. World J.* **2012**, *2012*, 104389. [CrossRef]

4. Damalas, D.; Megalofonou, P. Age-Dependent Habitat Identification of Mediterranean Swordfish: Application on Commercial Fishery Data and Potential Use in Fisheries Management. *Rev. Fish. Sci. Aquac.* **2014**, *22*, 257-273. [CrossRef]

5. ICCAT. Report of the 2020 ICCAT Mediterranean swordfish stock assessment meeting (22 May-6 June 2020). *ICCAT Collect. Vol. Sci. Pap.* **2020**, *77*, 179-316.

6. Young, J.; Drake, A.; Brickhill, M.; Farley, J.; Carter, T. Reproductive dynamics of broadbill swordfish, *Xiphias gladius*, in the domestic longline fishery off eastern Australia. *Mar. Freshw. Res.* **2003**, *54*, 315-332. [CrossRef]

7. Arocha, F.; Lee, D.W. Maturity at size, reproductive seasonality, spawning frequency, fecundity and sex ratio in the swordfish from the Northwest Atlantic. *Int. Comm. Conserv. Atl. Tunas Collect. Sci. Pap.* **1996**, *45*, 350-357.

8. Poisson, F.; Fauvel, C. Reproductive dynamics of swordfish (*Xiphias gladius*) in the southwestern Indian Ocean (Reunion Island). Part 2: Fecundity and spawning pattern. *Aquat. Living Resour.* **2009**, *22*, 59-68. [CrossRef]

9. Rey, J.C. Comentarios sobre las areas de reproducción del pez espada (*Xiphias gladius*) en el Atlantico y Mediterraneo. *ICCAT Collect. Vol. Sci. Pap.* **1988**, *27*, 180.

10. Romeo, T.; Perzia, P.; Esposito, V.; Malara, D.; Battaglia, P.; Consoli, P.; Canese, S.; Andaloro, F. Relationship between swordfish swimming behaviour and sea surface temperature in the central Mediterranean Sea during the reproductive period. *Mar. Biol. Res.* **2011**, *7*, 186-194. [CrossRef]

11. Tinti, F.; Addis, P.; Di Natale, A.; Garibaldi, F.; Piccinetti, C. The Italian annotated bibliography on Mediterranean spearfish (*Tetrapturus belone*, Rafinesque, 1810) and other billfish and spearfish species. *Collect. Vol. Sci. Pap. ICCAT* **2020**, *76*, 293-310.

12. Sella, M. Contrlbuto alia conoscenza della riprodutlone c della sviluppo del pesce spada (*Xiphias gladius*). *Mem. R. Com. Talassogr. Ital.* **1911**, *2*, 1-16.

13. Yasuda, F.; Kohno, H.; Yatsu, A.; Ida, H.; Arena, F.P.; Greci, F.L.; Taki, Y. Embryonic and early larval stages of the swordfish, *Xiphias gladius*, from the Mediterranean. *J. Tokyo Univ. Fish.* **1978**, *65*, 91-97.

14. Orsi Relini, L.; Palandri, G.; Garibaldi, F.; Cima, C.; Relini, M.; Torchia, G. Biological parameters of the Mediterranean swordfish derived from observations in the Ligurian Sea. *ICCAT Col. Vol. Sci. Pap.* **1999**, *49*, 397-406.

15. Anonymous. Report of the 2003 ICCAT Mediterranean swordfish stock assessment session. *Collect. Vol. Sci. Pap. ICCAT* **2004**, *56*, 789-837.
is unknown, and there is a need for more diet studies of large predatory fish. The spatial distribution and connectivity between swordfish spawning and putative nursery grounds during swordfish ontogeny also requires further investigation. Environmental shifts associated with climate change could unpredictably modify the reproduction dynamics, growth, recruitment and distribution of both species. This knowledge is important for the sustainable management of regional fisheries.

**Funding:** The Gen&Rec project ("Distribution and behaviour of swordfish spawners and juveniles in the vicinity of Corsica") is an IFREMER project funded by the European Union (European Maritime and Fisheries Fund—EMFF), grant number 18/2216415 and by the fishing industry (France Filière Pêche—FFP) grant number 20/1001220 conducted in close collaboration with French swordfish longline fishers with four partners (IRD, Stella Mare (Université de Corse Pasquale Paoli), CEFE and the fisheries committee of Corsica (CRPMEM Corsica).

**Institutional Review Board Statement:** No ethical parameters required as the gutted Dolphinfish (a marketable fish) used for the study was provided by skipper of the fishing boat.

**Data Availability Statement:** All data generated or analysed during this study are included in this published article.

**Acknowledgments:** All the fishers are acknowledged for their active participation in this project, particularly Jean-Toussaint Lucchini and his sons, owners of the vessel "Corail" which caught the dolphinfish concerned. I would like to thank the three reviewers and the Associate Editor for their comments that improved this manuscript.

**Conflicts of Interest:** The author declares no conflict of interest.

## References

1. Garibaldi, F. By-catch in the mesoplastic swordfish longline fishery in the ligurian sea (Western Mediterranean). *Collect. Vol. Sci. Pap. ICCAT* **2015**, *71*, 1495-1498.

2. Báez, J.C.; Camiñas, J.A.; Hernández, P.; Vasconcellos, M.; García-Barcelona, S.; Macías, D. North Atlantic Oscillation affects dolphinfish catch and bycatch in the Western Mediterranean Sea. *Reg. Stud. Mar. Sci.* **2020**, *36*, 101303. [CrossRef]

3. Macías, D.; Báez, J.C.; García-Barcelona, S.; Ortiz de Urbina, J.M. Dolphinfish Bycatch in Spanish Mediterranean Large Pelagic Longline Fisheries, 2000-2010. *Sci. World J.* **2012**, *2012*, 104389. [CrossRef]

4. Damalas, D.; Megalofonou, P. Age-Dependent Habitat Identification of Mediterranean Swordfish: Application on Commercial Fishery Data and Potential Use in Fisheries Management. *Rev. Fish. Sci. Aquac.* **2014**, *22*, 257-273. [CrossRef]

5. ICCAT. Report of the 2020 ICCAT Mediterranean swordfish stock assessment meeting (22 May-6 June 2020). *ICCAT Collect. Vol. Sci. Pap.* **2020**, *77*, 179-316.

6. Young, J.; Drake, A.; Brickhill, M.; Farley, J.; Carter, T. Reproductive dynamics of broadbill swordfish, *Xiphias gladius*, in the domestic longline fishery off eastern Australia. *Mar. Freshw. Res.* **2003**, *54*, 315-332. [CrossRef]

7. Arocha, F.; Lee, D.W. Maturity at size, reproductive seasonality, spawning frequency, fecundity and sex ratio in the swordfish from the Northwest Atlantic. *Int. Comm. Conserv. Atl. Tunas Collect. Sci. Pap.* **1996**, *45*, 350-357.

8. Poisson, F.; Fauvel, C. Reproductive dynamics of swordfish (*Xiphias gladius*) in the southwestern Indian Ocean (Reunion Island). Part 2: Fecundity and spawning pattern. *Aquat. Living Resour.* **2009**, *22*, 59-68. [CrossRef]

9. Rey, J.C. Comentarios sobre las areas de reproducción del pez espada (*Xiphias gladius*) en el Atlantico y Mediterraneo. *ICCAT Collect. Vol. Sci. Pap.* **1988**, *27*, 180.

10. Romeo, T.; Perzia, P.; Esposito, V.; Malara, D.; Battaglia, P.; Consoli, P.; Canese, S.; Andaloro, F. Relationship between swordfish swimming behaviour and sea surface temperature in the central Mediterranean Sea during the reproductive period. *Mar. Biol. Res.* **2011**, *7*, 186-194. [CrossRef]

11. Tinti, F.; Addis, P.; Di Natale, A.; Garibaldi, F.; Piccinetti, C. The Italian annotated bibliography on Mediterranean spearfish (*Tetrapturus belone*, Rafinesque, 1810) and other billfish and spearfish species. *Collect. Vol. Sci. Pap. ICCAT* **2020**, *76*, 293-310.

12. Sella, M. Contrlbuto alia conoscenza della riprodutlone c della sviluppo del pesce spada (*Xiphias gladius*). *Mem. R. Com. Talassogr. Ital.* **1911**, *2*, 1-16.

13. Yasuda, F.; Kohno, H.; Yatsu, A.; Ida, H.; Arena, F.P.; Greci, F.L.; Taki, Y. Embryonic and early larval stages of the swordfish, *Xiphias gladius*, from the Mediterranean. *J. Tokyo Univ. Fish.* **1978**, *65*, 91-97.

14. Orsi Relini, L.; Palandri, G.; Garibaldi, F.; Cima, C.; Relini, M.; Torchia, G. Biological parameters of the Mediterranean swordfish derived from observations in the Ligurian Sea. *ICCAT Col. Vol. Sci. Pap.* **1999**, *49*, 397-406.

15. Anonymous. Report of the 2003 ICCAT Mediterranean swordfish stock assessment session. *Collect. Vol. Sci. Pap. ICCAT* **2004**, *56*, 789-837.
16. Garibaldi, F.; Lanteri, L. Notes about a tagged:recaptured swordfish in the Ligurian Sea (Western Mediterranean). *Collect. Vol. Sci. Pap. ICCAT* **2017**, *74*, 1354–1361.

17. Tserpes, G.; Peristeraki, P.; Somarakis, S. On the reproduction of swordfish (*Xiphias gladius* L.) in the eastern Mediterranean. *Col. Vol. Sci. Pap. ICCAT* **2001**, *52*, 740–744.

18. Potoschi, A.; Cavallaro, G.; Sturiale, P.; Lo Duca, G. Eggs and larvae of swordfish, tuna and albacore caught in the Ionian Sea. *Biol. Mar. Medit.* **1994**, *1*, 119–124.

19. Millot, R.; Poisson, F.; Macías, D.; Saber, S.; Aiello, A.; Durieux, E.D.H. Reproductive traits and spawning activity of swordfish *Xiphias gladius* L. in the north-western Mediterranean Sea (Corsica). *Fish. Res.* **2023**, *267*, 106811. [CrossRef]

20. Palko, B.J.; Beardsley, G.L.; Richards, W.J. *Synopsis of the Biology of the Swordfish, Xiphias gladius Linnaeus*; US Department of Commerce, National Oceanic and Atmospheric Administration: Washington, DC, USA, 1981.

21. Arata, J.; George, F. A contribution to the life history of the swordfish, *Xiphias gladius* Linnaeus, from the South Atlantic coast of the United States and the Gulf of Mexico. *Bull. Mar. Sci.* **1954**, *4*, 183–243.

22. Jones, S. Notes on eggs, larvae and juveniles of fishes from Indian waters 1. *Xiphias gladius* Linnaeus. *Indian J. Fish.* **1958**, *5*, 357–361.

23. Nakamura, I. FAO Species Catalogue: Vol. 5. Billfishes of the World. An Annotated and Illustrated Catalogue of Marlins, Sailfishes, Spearfishes and Swordfishes Known to Date; FAO: Rome, Italy, 1985.

24. Nakamura, I. Systematics of the billfishes (Xiphiidae and Istiophoridae). *Publ. Seto Mar. Biol. Lab.* **1983**, *28*, 255–396. [CrossRef] [PubMed]

25. Sanzo, L. Uovo e larva di pesce-spada (*Xiphias gladius* L.). *Riv. Mens. Pesca Idrobiol.* **1910**, *12*, 206–209.

26. Govoni, J.J.; Laban, E.H.; Hare, J.A. The early life history of swordfish (*Xiphias gladius*) in the western North Atlantic. *Fish. Bull.* **2003**, *101*, 778–790.

27. McGowan, C. Differential development of the rostrum and mandible of the swordfish (*Xiphias gladius*) during ontogeny and its possible functional significance. *Can. J. Zool.* **2011**, *66*, 496–503. [CrossRef]

28. Govoni, J.J.; West, M.A.; Zivotofsky, D.; Zivotofsky, A.Z.; Bowser, P.R.; Collette, B.B. Ontogeny of squamation in swordfish, *Xiphias gladius*. *Copeia* **2004**, *2004*, 391–396. [CrossRef]

29. Di Natale, A.; De La Serna, J.M.; De Metrio, G.; Restrepo, V.; Srour, A.; Tserpes, G. On the reduction of juvenile swordfish catches in the Mediterranean. *Col. Vol. Sci. Pap. ICCAT* **2002**, *54*, 1529–1533.

30. Megalofonou, P.; Dean, J.; De Metrio, G.; Wilson, C.; Berkeley, S. Age and growth of juvenile swordfish, *Xiphias gladius* Linnaeus, from the Mediterranean Sea. *J. Exp. Mar. Biol. Ecol.* **1995**, *188*, 79–88. [CrossRef]

31. Tserpes, G.; Peristeraki, P.; Di Natale, A. Standardised catch rates for swordfish (*Xiphias gladius*) from the Italian and Greek fisheries operating in the central-eastern Mediterranean. *ICCAT Collect. Vol. Sci. Pap.* **2004**, *56*, 850–859.

32. ICCAT. *Report for Biennial Period, 2006–2007 Part II (2007)—Vol. 2*; SCRS: Madrid, Spain, 2008; 262p.

33. Peristeraki, P.; Lazarakis, G.; Skarvelis, K.; Kypraios, N.; Tserpes, G. Temporal growth differences of swordfish recruits in the eastern Mediterranean. *Int. Comm. Conserv. Atl. Tunas. Collect. Vol. Sci. Pap.* **2007**, *60*, 2063–2068.

34. ICCAT. Recommendation [16-05] by ICCAT replacing the recommendation [13-04] and establishing a multi-annual recovery plan for Mediterranean swordfish. In *ICCAT SCRS Report for the Biennial Period, 2018-17, Part 1 (2016)—Vol.1*; 2017. Available online: http://www.iccat.int (accessed on 10 July 2023).

35. Nunes, J.; Nunes, C.; Freitas, R.; Reis-Filho, J.; Loiola, M.; Sampaio, C. Feeding behaviour of the common dolphinfish *Coryphaena hippurus*: Older fish use more complex foraging strategies. *J. Mar. Biol. Assoc. U. K.* **2015**, *95*, 1277–1284. [CrossRef]

36. Whitney, N.; Taquet, M.; Brill, R.; Girard, C.; Schwieterman, G.; Dagorn, L.; Holland, K. Swimming depth of dolphinfish (*Coryphaena hippurus*) associated and unassociated with fish aggregating devices. *Fish. Bull.* **2016**, *114*, 426–434. [CrossRef]

37. Massutí, E.; Deudero, S.; Sánchez, P.; Morales-Nin, B. Diet and feeding of dolphin (*Coryphaena hippurus*) in western Mediterranean waters. *Bull. Mar. Sci.* **1998**, *63*, 329–341.

38. Massutí, E.; Morales-Nin, B. Seasonality and reproduction of dolphin-fish (*Coryphaena hippurus*) in the western Mediterranean. *Sci. Mar.* **1995**, *59*, 357–364.

39. Sinopoli, M.; Castriota, L.; Vivona, P.; Gristina, M.; Andaloro, F. Assessing the fish assemblage associated with FADs (Fish Aggregating Devices) in the southern Tyrrhenian Sea using two different professional fishing gears. *Fish. Res.* **2012**, *123–124*, 56–61. [CrossRef]

40. Gillespie, K.; Rosa, D.; Coelho, R.; Macías-López, A.; Gioacchini, G.; Busawon, D.; Poisson, F. Report of the 2021 ICCAT Swordfish Biology Workshop (Online, 22–26 March 2021). ICCAT Collective Volume of Scientific Papers. 2021. Available online: https://digital.csic.es/handle/10261/326270 (accessed on 28 December 2023).

41. Fierstine, H.L.; Voigt, N.L. Use of Rostral Characters for Identifying Adult Billfishes (Teleostei: Perciformes: Istiophoridae and Xiphiidae). *Copeia* **1996**, *1996*, 148–161. [CrossRef]

42. Rodriguez-Roda, J.; Howard, J.K. Presence of Istiophoridae along the South Atlantic and Mediterranean Coasts of Spain. *Nature* **1962**, *196*, 495–496. [CrossRef]

43. Massutí, E.; Morales-Nin, B. Reproductive biology of dolphin-fish (*Coryphaena hippurus* L.) off the island of Majorca (western Mediterranean). *Fish. Res.* **1997**, *30*, 57–65. [CrossRef]
16. Garibaldi, F.; Lanteri, L. Notes about a tagged:recaptured swordfish in the Ligurian Sea (Western Mediterranean). *Collect. Vol. Sci. Pap. ICCAT* **2017**, *74*, 1354–1361.

17. Tserpes, G.; Peristeraki, P.; Somarakis, S. On the reproduction of swordfish (*Xiphias gladius* L.) in the eastern Mediterranean. *Col. Vol. Sci. Pap. ICCAT* **2001**, *52*, 740–744.

18. Potoschi, A.; Cavallaro, G.; Sturiale, P.; Lo Duca, G. Eggs and larvae of swordfish, tuna and albacore caught in the Ionian Sea. *Biol. Mar. Medit.* **1994**, *1*, 119–124.

19. Millot, R.; Poisson, F.; Macías, D.; Saber, S.; Aiello, A.; Durieux, E.D.H. Reproductive traits and spawning activity of swordfish *Xiphias gladius* L. in the north-western Mediterranean Sea (Corsica). *Fish. Res.* **2023**, *267*, 106811. [CrossRef]

20. Palko, B.J.; Beardsley, G.L.; Richards, W.J. *Synopsis of the Biology of the Swordfish, Xiphias gladius Linnaeus*; US Department of Commerce, National Oceanic and Atmospheric Administration: Washington, DC, USA, 1981.

21. Arata, J.; George, F. A contribution to the life history of the swordfish, *Xiphias gladius* Linnaeus, from the South Atlantic coast of the United States and the Gulf of Mexico. *Bull. Mar. Sci.* **1954**, *4*, 183–243.

22. Jones, S. Notes on eggs, larvae and juveniles of fishes from Indian waters 1. *Xiphias gladius* Linnaeus. *Indian J. Fish.* **1958**, *5*, 357–361.

23. Nakamura, I. FAO Species Catalogue: Vol. 5. Billfishes of the World. An Annotated and Illustrated Catalogue of Marlins, Sailfishes, Spearfishes and Swordfishes Known to Date; FAO: Rome, Italy, 1985.

24. Nakamura, I. Systematics of the billfishes (Xiphiidae and Istiophoridae). *Publ. Seto Mar. Biol. Lab.* **1983**, *28*, 255–396. [CrossRef] [PubMed]

25. Sanzo, L. Uovo e larva di pesce-spada (*Xiphias gladius* L.). *Riv. Mens. Pesca Idrobiol.* **1910**, *12*, 206–209.

26. Govoni, J.J.; Laban, E.H.; Hare, J.A. The early life history of swordfish (*Xiphias gladius*) in the western North Atlantic. *Fish. Bull.* **2003**, *101*, 778–790.

27. McGowan, C. Differential development of the rostrum and mandible of the swordfish (*Xiphias gladius*) during ontogeny and its possible functional significance. *Can. J. Zool.* **2011**, *66*, 496–503. [CrossRef]

28. Govoni, J.J.; West, M.A.; Zivotofsky, D.; Zivotofsky, A.Z.; Bowser, P.R.; Collette, B.B. Ontogeny of squamation in swordfish, *Xiphias gladius*. *Copeia* **2004**, *2004*, 391–396. [CrossRef]

29. Di Natale, A.; De La Serna, J.M.; De Metrio, G.; Restrepo, V.; Srour, A.; Tserpes, G. On the reduction of juvenile swordfish catches in the Mediterranean. *Col. Vol. Sci. Pap. ICCAT* **2002**, *54*, 1529–1533.

30. Megalofonou, P.; Dean, J.; De Metrio, G.; Wilson, C.; Berkeley, S. Age and growth of juvenile swordfish, *Xiphias gladius* Linnaeus, from the Mediterranean Sea. *J. Exp. Mar. Biol. Ecol.* **1995**, *188*, 79–88. [CrossRef]

31. Tserpes, G.; Peristeraki, P.; Di Natale, A. Standardised catch rates for swordfish (*Xiphias gladius*) from the Italian and Greek fisheries operating in the central-eastern Mediterranean. *ICCAT Collect. Vol. Sci. Pap.* **2004**, *56*, 850–859.

32. ICCAT. *Report for Biennial Period, 2006–2007 Part II (2007)—Vol. 2*; SCRS: Madrid, Spain, 2008; 262p.

33. Peristeraki, P.; Lazarakis, G.; Skarvelis, K.; Kypraios, N.; Tserpes, G. Temporal growth differences of swordfish recruits in the eastern Mediterranean. *Int. Comm. Conserv. Atl. Tunas. Collect. Vol. Sci. Pap.* **2007**, *60*, 2063–2068.

34. ICCAT. Recommendation [16-05] by ICCAT replacing the recommendation [13-04] and establishing a multi-annual recovery plan for Mediterranean swordfish. In *ICCAT SCRS Report for the Biennial Period, 2018-17, Part 1 (2016)—Vol.1*; 2017. Available online: http://www.iccat.int (accessed on 10 July 2023).

35. Nunes, J.; Nunes, C.; Freitas, R.; Reis-Filho, J.; Loiola, M.; Sampaio, C. Feeding behaviour of the common dolphinfish *Coryphaena hippurus*: Older fish use more complex foraging strategies. *J. Mar. Biol. Assoc. U. K.* **2015**, *95*, 1277–1284. [CrossRef]

36. Whitney, N.; Taquet, M.; Brill, R.; Girard, C.; Schwieterman, G.; Dagorn, L.; Holland, K. Swimming depth of dolphinfish (*Coryphaena hippurus*) associated and unassociated with fish aggregating devices. *Fish. Bull.* **2016**, *114*, 426–434. [CrossRef]

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54. Peck, M.A.; Catalán, I.A.; Damalas, D.; Elliott, M.; Ferreira, J.G.; Hamon, K.G.; Kamermans, P.; Kay, S.; Kreiß, C.M.; Pinnegar, J.K.; et al. *Climate Change and European Fisheries and Aquaculture: 'CERES' Project Synthesis Report*; Universität Hamburg: Hamburg, Germany, 2020. [CrossRef]

**Disclaimer/Publisher's Note:** The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.
44. Moltó, V.; Hernández, P.; Sinopoli, M.; Besbes-Benseddik, A.; Besbes, R.; Mariani, A.; Gambin, M.; Alemany, F.; Morales-Nin, B.; Grau, A.M.; et al. A Global Review on the Biology of the Dolphinfish (*Coryphaena hippurus*) and Its Fishery in the Mediterranean Sea: Advances in the Last Two Decades. *Rev. Fish. Sci. Aquac.* **2020**, *28*, 376–420. [CrossRef]

45. Yabe, H.; Ueyanagi, S.; Kikawa, S.; Watanabe, H. Study on the life-history of the swordfish, *Xiphias gladius* Linnaeus. *Rep. Nankai Reg. Fish. Res. Lab.* **1959**, *10*, 107–150.

46. Pompanon, F.; Deagle, B.E.; Symondson, W.O.C.; Brown, D.S.; Jarman, S.N.; Taberlet, P. Who is eating what: Diet assessment using next generation sequencing. *Mol. Ecol.* **2012**, *21*, 1931–1950. [CrossRef] [PubMed]

47. Beckett, J.S. Biology of swordfish, *Xiphias gladius* L., in the Northwest Atlantic Ocean. In Proceedings of the International Billfish Symposium, Kailua-kona, HI, USA, 9–12 August 1972; Part 2. Review and contributed papers. U.S. Department of Commerce: Washington, DC, USA, 1974; pp. 105–106.

48. Dewees, C.M. Swordfish. In *California's Living Marine Resources and Their Utilization*; Leet, W.S., Dewees, C.M., Haugen, C.M., Eds.; California Sea Grant Extension Program: Davis, CA, USA, 1992; pp. 148–150.

49. Houde, E.D. Comparative growth, mortality and energetics of marine fish larvae: Temperature and implied latitudinal effects. *Fish. Bull.* **1989**, *87*, 471–495.

50. Orsi Relini, L.; Palandri, G.; Garibaldi, F. Reproductive parameters of the Mediterranean swordfish. *Biol. Mar. Mediterr.* **2003**, *10*, 210–222.

51. Garibaldi, F.; Palandri, G.; Orsi Relini, L. Documentazione della riproduzione del pesce spada in Mar Ligure. *Biol. Mar. Medit.* **2004**, *11*, 595–598.

52. Orsi Relini, L.; Palandri, G.; Garibaldi, F.; Relini, M.; Cima, C.; Lanteri, L. Large pelagic fish, swordfish, bluefin and small tunas, in the Ligurian Sea: Biological characteristics and fishery trends. *Chem. Ecol.* **2010**, *26*, 341–357. [CrossRef]

53. Hunter, J.R. Feeding ecology and predation of marine fish larvae. In *Marine Fish Larvae: Morphology, Ecology, and Relation to Fisheries*; Lasker, R., Ed.; Washington Sea Grant Program: Seattle, DC, USA, 1981; pp. 33–77.

54. Peck, M.A.; Catalán, I.A.; Damalas, D.; Elliott, M.; Ferreira, J.G.; Hamon, K.G.; Kamermans, P.; Kay, S.; Kreiß, C.M.; Pinnegar, J.K.; et al. *Climate Change and European Fisheries and Aquaculture: 'CERES' Project Synthesis Report*; Universität Hamburg: Hamburg, Germany, 2020. [CrossRef]

**Disclaimer/Publisher's Note:** The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.