Effect of non-additivity in mortality rates on predictions of potential yield of forage fishes
Forage fish is used to define highly productive, small and medium-sized low-nutrient pelagic fish such as anchovies, herring, sardines and krill that are preyed on by nutrient-rich species such as marine fish, mammals and birds. Most forage fish are very important in the marine food web as they feed on phytoplankton, zooplankton and sometimes early predator stages. They provide a major pathway for energy and nutrients to flow from low to high trophic levels, and several other species at this trophic level are as capable of directing energy flow as forage fish. Forage fish are also harvested for direct human consumption or to support a high-value fishery of fish eggs (such as herring) for the Japanese market, and thus have a direct impact on marine capture fisheries. A significant portion of the catch is 'reduced' into fishmeal and fish oil, which are primarily used as feed ingredients for aquaculture and terrestrial animals. Like many other products of natural systems, forage fish are produced by the oceans virtually free of charge. In addition to rising fishmeal and fish oil prices due to changes in the feed industry, the low cost of harvesting is very attractive to fishermen for short-term commercial gains. It has long been debated whether these stocks are more susceptible to invasion by fishing compared to fisheries. Global landings of important forage fish are estimated to be close to the average maximum sustainable yield (MSY). However, harvesting forage fish at conventional MSY levels in low-yielding years can lead to overfishing. Sustained high fishing quotas can also increase the risk of population collapse as productivity and stock abundance decline rapidly. Due to the uncertainties and inherent complexity of these highly dynamic species, the relationship between prey fish and their dependent predators is highly complex. There is no universal evidence that global declines in forage fish are affecting predator abundance. Nevertheless, the local and regional impacts of forage fish on predator reproduction, survival and population density should not be ignored. Even from this narrow perspective; forage fish encroachments can clearly pose a risk of long-term ecological and economic losses, especially given their role in supporting dependent predator fish. The risk of this stock collapse may increase as aquaculture production increases along with the aquaculture industry has doubled in size over the past decade and is expected to continue growing at a CAGR of 2.3% through 2026. The demand for fishmeal is increasing in response to the growth of aquaculture. Using both quantitative and qualitative evidence, this paper aims to describe the range of hidden non-market values in forage fish and to identify gaps for further research. Both directly and indirectly, we benefit from the presence of forage fish through a variety of uses. However, due to lack of data and gaps in existing research, the commercial importance of these species tends to override the wider benefits they bring to society and the environment. This paper aims to identify all the multiple beneficiaries of forage fish and present global values covering the benefits of different categories using both quantitative and qualitative methods.