# In situ multi-indicators for the management of pelagic fish resources in shallow water lagoons

Patrice Brehmer, Francis Laloë, Thang Do Chi, Thierry Laugier, David

Mouillot, Pablo Ivan Caballero Pinzon

► To cite this version:

Patrice Brehmer, Francis Laloë, Thang Do Chi, Thierry Laugier, David Mouillot, et al. In situ multi-indicators for the management of pelagic fish resources in shallow water lagoons. 6ième forum Halieumétrique, Jun 2003, Montpellier, France. Association Française d'Halieumétrie, p.197-197, 2003, 6ième forum Halieumétrique, Montpellier, 24 - 26 juin 2003. hal-04402944

HAL Id: hal-04402944

https://hal.science/hal-04402944v1

Submitted on 19 Jan 2024

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<figure><img src="image_1.png" /></figure>

# In situ multi-indicators for the management of pelagic fish resources in shallow water lagoons

Brehmer Patrice, Laloe Francis, Thang Do Chi, Thierry Laugier, David Mouillot, Pablo Caballero

## Monitoring three shallow water Mediterranean lagoon
at several levels of eutrofication. Fishing operations were carried out during autumn 1999. Commercial catches were monitored the same year using monthly surveys on the three lagoons. In autumn, the fish flow and the current amplitude inside the lagoon channel have been recorded using the split beam echosounder. Daily cycles were considered in September, October and November which are known as a period of occurrence of migration and transfer across the channel between the sea and the lagoon. Additional multi-beam high-resolution sonar data were recorded simultaneously with echo sounder on two lagoons in September and November.

<figure><img src="image_2.png" /><figcaption>Variation of temperature and salinity, environmental data...</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Lagoon channel « grau » and mobile acoustic laboratory</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>High resolution sonar (455 kHz) Data</figcaption></figure>

Infobancs acquisition menu

school

Zoom on a small fish

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

<figure><img src="image_6.png" /><figcaption>Split beam echo sounder Data</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fishing Data (commercial and scientific)</figcaption></figure>

<figure><img src="image_8.png" /></figure>

## Hydrodynamics status of the channel

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

General Linear Model applied to current flow data analysed by Fourier series

The results show that the data fit well with the model. The highest variations of the currents speed are recorded in Prevost lagoon, the lowest were recorded in Ingril lagoon

## Fish fluxes across the channel

<figure><img src="image_13.png" /></figure>

The split-beam echosounder allows to detect individual fish and an « echo counting » of the number of fish which cross the channel in both directions (up and downstream).

The upper set of graphics shows fish movements according to the time of the day. A continuous monitoring using echo counting allows to evaluate the biomass transfer between the lagoon and the sea but also their swimming speed for large target, small target generated wrong absolute speed measurements.

The histograms show the schools fluxes. Upstream and downstream fluxes are much higher than net fluxes (upstream minus downstream fluxes), which suggest the « multi-pass hypothesis ». Therefore, the net fluxes seem to be an indicator of the real biomass transfer between the lagoon and the sea. Sonar record allows also kinematics studies, here we show two kind of swimming behaviour: very straight or more sinuous).

Both indicators and movement of fishes according to the time of the day can be assessed separately with reference to the schools and to the individual fish.

## Global eutrofication state

<figure><img src="image_14.png" /></figure>

Five levels are fixed for the quality of the : water column, phytoplankton, macrophyte, biological potentiality and sediment

The Web based Lagoon Monitoring Network (RSL) allows inter-comparisons between 22 lagoons of the Languedoc-Roussillon Region (Ifremer-DEL)

## Fish species diversity and CPUE

<table><thead><tr><th>Setember 1999</th><th>CPUE Total</th><th>Dicentrarchus labrax</th><th>Mugilidae</th><th>Sparus aurata</th><th>Atherina boyeri</th><th>Anguilla anguilla</th></tr></thead><tbody><tr><td>Or</td><td>575,32</td><td>0,00</td><td>3,66</td><td>3,71</td><td>7,34</td><td>200,45</td></tr><tr><td>Prevost</td><td>219,53</td><td>9,30</td><td>16,68</td><td>10,71</td><td>0,57</td><td>128,68</td></tr><tr><td>Ingril</td><td>325,57</td><td>118,00</td><td>3,99</td><td>0,41</td><td>2,40</td><td>158,74</td></tr></tbody></table>

<table><thead><tr><th>Sept. 1999</th><th>Ingril</th><th>Or</th><th>Prevost</th></tr></thead><tbody><tr><td>Nb. Species</td><td>15</td><td>16</td><td>13</td></tr><tr><td>Delta Plus</td><td>87,86</td><td>90,42</td><td>92,95</td></tr><tr><td>Lambda Plus</td><td>215,65</td><td>262,33</td><td>46,43</td></tr></tbody></table>

<figure><img src="image_15.png" /></figure>

The diversity are similar for Or and Prevost lagoon, higher in Ingril. The CPUE revel that the more abundant species by lagoon.

Monthly commercial fishing data allow to follow the evolution of caught. It permit also to discriminate species caught by modality of size (large/small)

## Fish behaviour

<figure><img src="image_16.png" /></figure>

Data on 93 881 individual fish were recorded during 9 diel cycles across three lagoon channels. Acoustics data allow to assess indicators of fish behaviour, i.e. :

- flux direction: upstream and downstream

- spatial location in the channel lagoon

- period of observation (temporal variation in lagoon water temperature and salinity)

- diel behaviour: night, day, transition period, etc.

## Fish (TS) and school (Dimensions) size

<figure><img src="image_17.png" /></figure>

Split beam echo sounder allows to measure the Target Strength of an echo of individual fish. The multi-beam sonar gives the size of the fish school recorded :

(i) the Target strength (dB) of the individual fish which can be related to fish length

(ii) the school dimensions across (Cw) and along (Lw) the acoustic beams, and their shape (Lw/Cw)

## Discussion-Conclusion

Data from acoustics surveys and fishing operations are an aid to the analysis of anthropogenic effects on the management of the pelagic fish population of shallow water lagoons. For such purpose several indicators of the state of the fish resources are performed. The method is versatile as regards its uses. The impact of eutrofication on fish resources can be measured at the level of a shallow water lagoon using specific indicators. *In situ* behaviour of individual fish and schools can be approached using comparative approaches. Improvement of the accuracy of biomass estimates using echo-integration or fishing methods required improving knowledge on fish behaviour as well as on lagoon ecosystem functioning. In such complex system often declared as protected area and important nurseries of numerous fishes, the variety of available indicator allows now the fisheries scientist to propose adapted sensitive responses to local authorities for « decision making ».