What is affecting those tasty little fish?
The coast of South Africa hosts an amazing variety of fish species. If you ask me to choose only one or two among all of them (please, do not do that), I will have to say that anchovy (Engraulis encrasicolus) and sardine (Sardinops sagax) are the most important ones. Because they are yummy, yes, but for other reasons too. Let me explain.
Anchovy and sardine are the most abundant species of the so-called group of forage fish or small pelagic fish in South Africa and all around the world’s oceans. In some regions, like the Mediterranean Sea, anchovy and sardine alone can make up to 50% of the total fish biomass. These fishes’ ecological relevance is enormous: they connect the lower trophic levels of the marine food chains (that is, the plankton) with marine top predators such as large fish, seabirds and many marine mammal species. In addition, the economic importance of anchovy and sardine stands out, as they can sum up to half of all the fish catches in the world.
Particularly, in the South African coast anchovy and sardine represent the main prey for the endangered African penguins, cape gannets, dolphins, whales, fur seals and even, depending on the season, sharks and other fishes. In addition, in the cold waters of the South Atlantic Ocean, off the shore of beautiful Cape Town, we find one of the most important pelagic fishery in Africa, which targets both anchovy and sardine.
A few years ago, researcher detected a shift in the distribution and main spawning areas for these two fish species. Anchovy and sardine appeared to be moving eastwards, reaching for the warmer waters of the Indian Ocean. Although the reasons for this shift are not completely clear yet, most experts attribute it to either climate change or fishing pressure, or both. The consequences of this shift, nevertheless, are likely to be of extreme relevance for the main anchovy and sardine predators and the fisheries that rely on them.
Algoa Bay is located in the southeast of South Africa and has relatively small populations of anchovy and sardine. Given the recent changes in the distribution of these species from the Atlantic Ocean to the southeast of South Africa, we should put a greater attention on the fluctuations of these populations in this region. Since the fish larvae are particularly sensitive to environmental and anthropogenic variables, the study of early life stages is key to understand how the fish populations vary.
In a study published a few days ago*, my colleagues and I compared the responses of the fish larvae to different factors before and after a long drought period in 2008-2010 in Algoa Bay. In the study, we generated different statistical models that included one or more of the variables, and the possible interactions among the variables. In particular, we investigated how wind speed and direction, rainfall, water temperature, chlorophyll a (as an indicator of productivity), food availability (in terms of zooplankton density, the main prey for small pelagic fish), and densities of larval fish are modulating the abundance of anchovy and sardine larvae.
We suggest, for example, that climatic oscillations can modify the strength and the direction of the dominant winds in the region, and consequently, the growth of the larval fish might be seriously limited. Therefore, fewer individuals will reach the adult stage, which could ultimately put the fish population under threat. With the increasingly grave effects of global change, there is an urge to comprehend which, and how, environmental factors influence the early stages of key species like sardine and anchovy.
In addition, the fact that the larvae of both species anchovy and sardine are found in Algoa Bay during the same season adds extra pressure to the viability of their populations. In most areas of the world where anchovy and sardine species co-exist, the larvae of these species are rarely found together. However, in Algoa Bay competition for food might be an important factor limiting the abundance of anchovy and/or sardine larvae.
Our results also highlight the fact that larval fish densities can be modelled –and to some extent predicted- using just a limited number of environmental predictors. This modelling approach is particularly useful when studying marine environments because of the difficulties associated to simulating the conditions of such a vast and variable ecosystem in the lab. In addition, these models can provide us with indicators to evidence-based decision for the conservation and management of aquatic resources.
It is unlike that we will see the anchovy and sardine populations in South Africa disappear. However, it is clear that the great relevance of these species for the environment and for the economic stability of fishing communities around the Globe calls for further research.
*Costalago, D., Potter, P., Pattrick, P., & Strydom, N. A. (2017). Influence of environmental variables on the larval stages of anchovy, Engraulis encrasicolus, and sardine, Sardinops sagax, in Algoa Bay, South Africa. Environmental Biology of Fishes, 1-12.
Other interesting references:
• Checkley Jr, D. M., Asch, R. G., & Rykaczewski, R. R. (2017). Climate, anchovy, and sardine. Annual review of marine science, 9, 469-493. • Coetzee, J. C., Van der Lingen, C. D., Hutchings, L., & Fairweather, T. P. (2008). Has the fishery contributed to a major shift in the distribution of South African sardine?. ICES Journal of Marine Science, 65(9), 1676-1688. • McInnes, A. M., Ryan, P. G., Lacerda, M., Deshayes, J., Goschen, W. S., & Pichegru, L. (2017). Small pelagic fish responses to fine-scale oceanographic conditions: implications for the endangered African penguin. Marine Ecology Progress Series, 569, 187-203. • Mhlongo, N., Yemane, D., Hendricks, M., & Lingen, C. D. (2015). Have the spawning habitat preferences of anchovy (Engraulis encrasicolus) and sardine (Sardinops sagax) in the southern Benguela changed in recent years?. Fisheries Oceanography, 24(S1), 1-14.
