A team of French researchers1 has studied the decline in catches of small pelagic fish (sardines, anchovies) by French small-scale fishermen in the north-western Mediterranean over the last ten years. This decline is thought to have two causes: a variation in the size and biomass of their prey (plankton) due to changes in the intensity of winter convection, and an increase in the biomass of their predators (horse mackerel, mackerel, poutassous, etc.), whose juveniles have more meso-zooplankton to consume.
The aim of this study, carried out using the OSMOSE-GoL integrated ecosystem model by researchers from MIO, AMAP and MARBEC, is to gain a better understanding of why, over the last ten years or so, there has been a more or less regular decline in catches of small pelagic fish (sardines, anchovies) by French artisanal fishermen in the north-western Mediterranean, and more particularly in the Gulf of Lion.
The model used in this study is an end-to-end model, i.e. it provides a numerical representation of the food web, from plankton to certain species of predatory fish or cephalopods, in a given climate and fishing context. The species represented are those of economic or ecological interest, in this case around ten teleost and cephalopod species representing around 70 % of catches in the Gulf of Lion. The particularity and originality of the model used lies in its ability to operate a totally bidirectional and dynamic coupling between the upper and lower trophic levels represented.
The results show that the diet of small pelagic planktivorous fish (anchovies, sardines, sprats) consists mainly of microplankton (20-200 µm) and mesozooplankton (200 µm - 2 cm) with a maximum predation rate corresponding, respectively, to 20 and 30 % of the natural mortality rates of these categories of plankton. These predation rates may appear low in absolute terms, but they are sufficient to have a significant impact on annual and sub-seasonal variations in the abundance of plankton and their teleost predators. Commercial fish landings are also affected, although in the model the fishing effort is assumed to be constant. The zone of influence of the Rhône plume and the regions affected by intense eddy activity (i.e. in the north-western part of the Gulf) are prime plankton predation zones because they are places where plankton is very abundant.
The model highlights the bottom-up (from the bottom of the food web) and top-down (from the top of the food web) controls within the ecosystem and also enables a more in-depth analysis of its complex functioning.
The four simulated years show winter convections of varying intensity, leading to significant variations in the size spectra and biomass of plankton in early spring. They also show that these variations in size structure could be the cause of a change in the diet of small pelagic fish (bottom-up control). The result would be a reduction in the average size of these fish and in their exploitable biomass, which corroborates the curve in landings observed in the field over several years.
If the biomass of plankton and planktivorous fish decreases, we might expect a decrease in the predators of these fish. However, over the duration of the simulation, we see a sharp increase in the biomass of predators and therefore in top-down control. In fact, the biomass of planktivorous species is decreasing, leading at the same time to a reduction in their predation on mesozooplankton. This category of plankton can therefore be consumed more by the juveniles of predators such as horse mackerel, mackerel, poutassous or cephalopods, which stimulates their recruitment. As a result, the biomass of these predators increased by 38 %.
In conclusion, small pelagic fish are subject to a double pressure linked to a change in the size and biomass of their prey and an increase in the biomass of their main predators.
The environmental changes (pH, temperature and chemical composition of marine water) currently observed in the Mediterranean are occurring at a much faster rate than those in the global ocean and are set to accelerate further over the next two decades at least. In this context, marine hydrodynamics (e.g. eddies, deep convection, etc.) should be profoundly affected, leading to a spatial and seasonal redistribution of nutrient salts in the north-western Mediterranean. In view of the results obtained in this study, we can therefore wonder about the operating trajectory that the marine pelagic ecosystem will take in the short and medium term in this region. To answer this question, the end-to-end model that has been developed will have to be compared with longer data series that incorporate variations in fishing effort, the observed increase in biomass due to the migration of predators (e.g. bluefin tuna), changes in nutrient inputs from rivers (e.g. the Rhône) and environmental variations (temperature, wind regime, stratification, etc.) linked to global change.
Highlighting the effects of trophic cascades within the modelled ecosystem and the differences in abundance between bidirectional and unidirectional couplings between the low and high trophic level models. The arrows indicate changes in biomass at each trophic level. The size of the circles is proportional to the average biomass. The grey circles correspond to the biomass in the unidirectional mode (plankton is eaten by planktivorous species) of coupling, while the black circles correspond to those in the bidirectional mode (plankton is eaten by planktivorous species which induce a corresponding mortality in the plankton).
Source
Implementation of an end-to-end model of the Gulf of Lions ecosystem (NW Mediterranean Sea). II. Investigating the effects of high trophic levels on nutrients and plankton dynamics and associated feedbacks, Frédéric Diaz, Daniela Bănaru, Philippe Verley, Yunne-Jai Shin, Ecological Modelling, Vol. 405, 1 August 2019
https://www.sciencedirect.com/science/article/pii/S0304380019301693?via%…
This manuscript is linked to the following research article: "Implementation of an end-to-end model of the Gulf of Lions ecosystem (NW Mediterranean Sea). I. Parameterization, calibration and evaluation", Bănaru et al. (2019), Ecological Modelling 401, 1-19,
https://doi.org/10.1016/j.ecolmodel.2019.03.005
Notes
The laboratories involved are: the Institut méditerranéen d'océanographie (MIO/PYTHÉAS, CNRS / Université de Toulon / IRD / Aix-Marseille Université (AMU)), the Botanique et modélisation de l'architecture des plantes et des végétations laboratory (AMAP, CNRS / INRA / CIRAD / Université Montpellier / IRD) and the Centre pour la biodiversité marine, l'exploitation et la conservation (MARBEC, Université Montpellier / CNRS / IRD / Ifremer).
Contact
Frédéric Diaz
MIO/PYTHÉAS
04 86 09 05 64
frederic.diaz@mio.osupytheas.fr