Small pelagic fish dynamics: A review of mechanisms in the Gulf of Lions

Abstract

Around 2008, an ecosystem shift occurred in the Gulf of Lions, highlighted by considerable changes in biomass and fish mean weight of its two main small pelagic fish stocks (European anchovy, Engraulis encrasicolus; European sardine, Sardina pilchardus). Surprisingly these changes did not appear to be mediated by a decrease in fish recruitment rates (which remained high) or by a high fishing pressure (exploitation rates being extremely low). Here, we review the current knowledge on the population's dynamics and its potential causes. We used an integrative ecosystem approach exploring alternative hypotheses, ranging from bottom-up to top-down control, not forgetting epizootic diseases. First, the study of multiple population characteristics highlighted a decrease in body condition for both species as well as an important decrease in size resulting both from a slower growth and a progressive disappearance of older sardines. Interestingly, older sardines were more affected by the decrease in condition than younger ones, another sign of an unbalanced population structure. While top-down control by bluefin tuna or dolphins, emigration and disease were mostly discarded as important drivers, bottom-up control mediated by potential changes in the plankton community appeared to play an important role via a decrease in fish energy income and hence growth, condition and size. Isotopic and stomach content analyses indicated a dietary shift pre-and post-2008 and modeled mesozooplankton abundance was directly linked to fish condition. Despite low energy reserves from 2008 onwards, sardines and anchovies maintained if not increased their reproductive investment, likely altering the lifehistory trade-off between reproduction and survival and resulting in higher natural mortality. The current worrying situation might thus have resulted from changes in plankton availability/diversity, which remains to be thoroughly investigated together with fish phenotypic plasticity.

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

“…On the whole the size-based predation pattern appeared to change towards smaller individuals eating smaller zooplankton and microphytoplankton when the intensity of convection increases. This pattern is in line with recent observations showing that European pilchard and European anchovy were consuming smaller prey than in previous studies (Le Bourg et al, 2015), and is probably one of the main factors explaining the decrease in their body condition and the current small pelagic fisheries crisis (Brosset et al, 2016;Saraux et al, 2018). Diaz et al Ecological Modelling xxx (xxxx) xxx-xxx…”

“…Our study shows that mesoscale structures and Rhone river inputs may thus play an important role in fish recruitment in the GoL and may be related to recent changes observed in small pelagic fish stocks. This seems to corroborate the hypothesis of a bottom-up control of their stocks (Le Bourg et al, 2015;Saraux et al, 2018).…”

“…Forage species, such as European pilchard and European anchovy, representing usually more than 50% of the total catches in this area, are one of the most important prey groups, being a major trophic link between plankton and top predators as some fish species, seabirds and mammals (Bănaru et al, 2013). However, over the last ten years, and despite the decrease in fishing pressure, the mean size and body condition of European pilchard and European anchovy have declined (Van Beveren et al, 2014;Saraux et al, 2018). Recent studies on their diet (Le Bourg et al, 2015) showed that they probably consume smaller prey than in the past, and that they maybe in competition with the European sprat, a non-commercial planktivorous fish which has strongly increased in biomass during recent years in this area.…”

Changes in the spawning habitat of two small pelagic fish in the Northwestern Mediterranean

“…Previous studies conducted in the area showed that under high‐temperature conditions, the abundance of different zooplanktonic groups, indicator of larval food resources, was very low (Atienza, Sabatés, Isari, Saiz, & Calbet, ; Maynou et al, ), which would negatively affect fish larval abundance. In addition to trophic limitation of fish larvae, changes in plankton composition and abundance may have important effects on the condition of juvenile and adult fishes, determining demographic parameters such as reproduction, growth, and mortality, and these changes have been proposed as important drivers of small pelagic population variability in the NW Mediterranean (Brosset et al, ; Coll et al, ; Saraux et al, ). Another indirect factor related to changing plankton dynamics is the increasing abundance and changes in gelatinous plankton composition that have been recorded in the NW Mediterranean (Guerrero, Gili, Grinyó, Raya, & Sabatés, ; Molinero et al, ).…”

Food in the Sea: Size Also Matters for Pelagic Fish

“…Until the mid-2000s, sardine (Sardina pilchardus) and anchovy (Engraulis encrasicolus) represented up to 50% of the total annual landings (around 15,000 tons) in the Gulf of Lions (Bǎnaru et al, 2013). Landings of sardines and anchovies have, however, decreased sharply since 2008, reaching the lowest levels recorded in 150 years (around 3,000 tons), although population abundance remains high (Van Beveren et al, 2016a; General Fisheries Commission for the Mediterranean [GFCM], 2017b;Saraux et al, 2019). Declines in landings of small pelagic fishes are typically a result of recruitment failure and/or overfishing (Schwartzlose et al, 1999).…”