DOI: 10.5061/dryad.g79cnp5zj
title: Parasite scars: The impact of salmon lice injury on sea trout populations

description: Spillback effects of the parasitic salmon louse from fish farms to wild
 fish are a critical environmental concern for salmon-producing countries.
 This ectoparasite causes significant physiological damage to wild
 salmonids. However, strong empirical links between injuries and abundance
 lice has not been established in situ. This study empirically establishes
 a strong association between dorsal fin damage in sea trout (Salmo trutta)
 and infestation with mobile stages of salmon lice at a marine site. We
 further assessed the prevalence of such damages on returning sea trout to
 the spawning grounds through snorkelling observations in 16 rivers in
 Hardangerfjord, western Norway, from 2007-2021. The prevalence of injuries
 decreased with distance from the fjord outlet, from ~70% in middle and
 outer parts to ~10% in the inner parts. Additionally, a negative
 correlation was found between sea trout population size and the proportion
 of the population with dorsal fin damage. The results demonstrate that
 salmon lice inflict high levels of injury on significant portions of sea
 trout populations in areas with intensive fish farming, leading to poorer
 population status in affected populations.
description: Data from sea trout with sea lice were collected from marine
 survey data on sea trout caught in a custom designed trap-net (Barlaup et
 al. 2013) in Herdlafjorden in the years 2011, 2013, 2014, and 2015 as a
 part of the national surveillance of salmon lice on wild salmonid fish
 (NALO). The traps were set up from the end of May to the end of start of
 July. In 2011, 2013 and 2014 fish were euthanized and placed in bags
 labelled with the date and frozen. The fish were later defrosted in the
 lab, weighed (wet mass, ± 1 g), and measured (total length, ± 1mm) and
 different stages of sea lice on each fish counted according to Hamre et
 al. (2013). In 2015, lice counts were instead conducted on live trout in
 the field. Each fish was placed in a bath of a mixture of ethanol and
 clove oil (1 part clove oil to 9 parts 70% ethanol) mixed in seawater at a
 ratio of 4.5 ml mixture per 10 liters of water. This was done to calm the
 fish so that lice could be counted (Figure 1). Once they had recovered
 from sedation, they were released back into the sea. Reporting dorsal fin
 damage is not part of the routine survey data collected in the survey, but
 the personnel at these locations were specifically asked to define whether
 or not the fish had dorsal fin damage or other damages. The dorsal fin was
 classified as either “no damage”, “partial missing” or “missing” (see
 figure 1 for examples). Spawning count surveys are
 conducted annually in most of the rivers in Hardangerfjorden including
 2007-2021. During spawning count surveys, personnel with snorkel using
 either a dry suit or a wet suit swim down the river in parallel (varying
 from 1- 4 people in a row) counting fish and classifying them according to
 size categories, species (trout or salmon) and origin (hatchery
 fish/escaped farmed fish/wild fish; Mahlum et al. 2019; Skoglund et al.
 2021). Dorsal fin lesions were noted only on sea trout when fish are
 appropriately close and the water visibility allows it. This means that
 the fish observed for lesions are only a subsample of the fish in the
 river. The lesions are classified as a binomial outcome– damaged dorsal
 fin or not. Examples of a dorsal fin with a typical observable damaged
 dorsal fin is given in figure 1d. Dorsal fin damage classifications were
 only done by personnel with several years of experience snorkelling to
 maintain internal consistency within the dataset. To assess potential bias
 in observation we plotted the proportion of fish observed against the
 total number of fish observed for each river (Appendix A1), with the
 expectation that the proportion would correlate negatively with number of
 fish observed if there was an observation bias. No such pattern was
 found.
description: # Parasite scars: The impact of salmon lice injury on sea trout
 populations ## Dataset Summary: This dataset explores the relationship
 between parasitic salmon lice (*Lepeophtheirus salmonis*) infestations and
 dorsal fin injuries in sea trout (*Salmo trutta*) populations in
 Hardangerfjord, Norway. It includes data from marine and spawning surveys
 collected over a 15-year period (2007-2021). The study assesses the impact
 of sea lice on trout populations, focusing on dorsal fin injuries as an
 indicator of damage, and correlates these injuries with lice abundance,
 geographic distribution, and population size. ## Data Files: The dataset
 consists of two primary CSV files: ### dorsalfin\_ surveillance\_Data.csv:
 Contains data from marine surveys on sea trout, including lice counts and
 dorsal fin damage assessments. #### Columns: Year: Year of the survey.
 Date: Date of data collection. Length (cm): Total length of the fish,
 measured in centimeters. Mass (g): Total weight of the fish, measured in
 grams. Damaged_dorsalfin: Indicates the state of dorsal fin damage
 (categorical: "No Damage", "Partial Missing",
 "Missing"). Total_lice: Total number of lice observed on each
 fish. mature: Number of mature lice found on the fish. mobile_non_mature:
 Number of non-mature but mobile lice. attached: Number of lice in the
 attached stage. mobile: Total number of mobile lice (both mature and
 non-mature). ### dorsalfin\_spawning\_data.csv: Contains data from
 spawning ground surveys in rivers of Hardangerfjord. #### Columns:
 Rivernr: River number, serving as a unique identifier for each river.
 River: Name of the river where observations were made. Year: Year of the
 survey. distance_to_sea: Distance from the fjord outlet (in kilometers).
 no_damage: Number of sea trout observed with no dorsal fin damage. damage:
 Number of sea trout observed with dorsal fin damage. total_obs: Total
 number of sea trout observed during the survey. Rivername: Name of the
 river (redundant with River). Areal_m2: The surface area of the river in
 square meters. Sum_seatrout: Total number of sea trout in the river (sum
 across the survey period). density: Density of sea trout in the river
 ((Sum_seatrout/Areal_m2)*10000) prop: Proportion of the population
 observed with dorsal fin damage (calculated as damage / total_obs).
 **Missing data are represented as NA.** ### Abbreviations: ID: Identifier
 for individual fish. NA: Not available (missing data). ### Units: Fish
 Length: Measured in centimeters (cm). Fish Mass: Measured in grams (g).
 Distance From Fjord: Measured in kilometers (km). ## Methods: ### Marine
 Survey Data Collection (2011, 2013, 2014, 2015): Sea trout were caught in
 a trap-net in Herdlafjorden. In 2011, 2013, and 2014, fish were
 euthanized, measured, weighed, and examined for lice counts and dorsal fin
 damage. In 2015, live fish were anesthetized with clove oil and ethanol,
 lice were counted, and dorsal fin damage was assessed. Fish were then
 released back into the sea. ### Spawning Survey Data Collection
 (2007-2021): Snorkeling surveys were conducted in 16 rivers, with
 personnel counting sea trout and noting dorsal fin damage. Fish were
 classified by size, species, and origin (wild or farmed). Dorsal fin
 damage was noted only for trout when conditions allowed close
 observations. ### Classification of Dorsal Fin Damage: "No
 Damage", "Partial Missing", or "Missing" based on
 visual assessment by trained personnel. ## Data Source: Data were
 collected through field surveys in Hardangerfjord and Herdlafjorden,
 Norway. Methods are based on national protocols for sea lice surveillance
 and fish damage assessment. ## Citation: If using this dataset, please
 cite it as follows: Vollset, K., Lehmann, G. B., Skår, B., Ulgenes, I.,
 Lennox, R. J., & Skoglund, H. (2024). Parasite scars: The impact of
 salmon lice injury on sea trout populations. Dryad.[
 https://doi.org/10.5061/dryad.g79cnp5zj](https://doi.org/10.5061/dryad.g79cnp5zj) ## Licensing: This dataset is licensed under the Public Domain.