Forage fish, small pelagic fisheries and recovering predators: managing expectations

Small pelagic fishes (i.e. sardines [Sardinops spp.] and anchovies [Engraulis spp.]; hereafter ‘forage fish’) support very large fisheries globally, but in recent years, the catches of sardine and anchovy off California have been very low. Sardine catches were two orders of magnitude lower, and anchovy catches are an order of magnitude lower than the historical maximum catch rates in this region (NMFS, 2009; Hill, Crone & Zwolinski, 2017). Declines in small pelagic fish biomass have occurred in spite of precautionary management including: very low exploitation rates, an environmentally informed harvest control rule for sardine and generous reserve thresholds to provide a buffer for forage and stock recovery (NMFS, 2009; Hill et al., 2017). Clearly something is missing. In addition to commercial harvest, the non-commercial value of forage fishes in the California Current System is a fundamentally important resource base for fish, mammal and seabird predators (Szoboszlai et al., 2015). In recent years, the largest removals of forage fish in the California Current System are by demersal fish, marine mammals, seabirds (Thayer, Szoboszlai & Sydeman, 2017) and then by fisheries, arranged in order of magnitude. Forage fish removals by commercial fisheries off California are currently <15% of consumption by marine mammals. While there is considerable uncertainty in this estimate, it is likely to be approximately correct. For example, California sea lion [Zalophus californianus], once a highly depleted marine mammal species, is now above 250 000 individuals (Laake et al., 2018). Using a conservative, non-pup provisioning ration of 5 kg/individual/ day (Costa, Antonelis & DeLong, 1991; Williams et al., 2007), the forage fish harvest would be 446 000 mt (metric tons) /year. If even a tenth is anchovy, this is greater than human harvest, without considering other marine mammals, marine birds or higher trophic-level fishes. The sardine fishery off California has been closed since 2015 following a stock assessment (Hill et al., 2015) estimating biomass less than the reserve threshold of 150 000 mt; a limit set to protect the sardine stock. Anchovy have been fished in the last decade (2006–2016) in the absence of a catch limit, although there is a trigger threshold, and catches have been very low (1020–17 284 mt; Lowther & Liddel, 2016). In 2016, a catch limit for anchovy was set at 25 000 mt, but actual catches (8366 mt) remained well below the catch limit. Despite this, the catch limit was challenged by environmental NGOs seeking to close the fishery, and the District Court ruled against the National Marine Fisheries Service (NMFS) for failing to apply ‘best available science’ (MacCall et al., 2016) when setting the catch limit. The court ruling, which at the time of writing may still be appealed, required NMFS to revise the catch limit. NMFS relied on historical data to establish the initial catch limit, rather than the analysis by MacCall et al. (2016), which had some problems – first, the spatial coverage was inadequate for an accurate biomass estimate (FRD, 2016), and second, the anchovy biomass estimate by MacCall et al. (2016) was less than the estimated consumption by marine mammals alone, which should not be the case unless the turnover of anchovy was phenomenally high. The timing of the lawsuit was unfortunate, in that NMFS acoustic-trawl survey data available shortly after the court case estimated the anchovy biomass at 150 000 mt (Zwolinski et al., 2017). At a biomass of more than 150 000 mt, a catch of <25 000 mt would still be precautionary for these fast growing fish. The focus on commercial fishery catches ignores an important source of sardine and anchovy mortality. Recovery of marine mammal populations implies large changes in natural mortality of forage fish. Following the introduction of the Marine Mammal Protection act in 1972, NMFS has monitored mammal populations in the California Current System and developed criteria to evaluate population recovery. It is now apparent that some of these populations, such as the California sea lion have recovered and are approaching carrying capacity (Laake et al., 2018). When food limitation contributes to mortality, populations can be expected to exhibit density-dependent mortality, or difficulty in adequately feeding their young (McClatchie et al., 2016; Wells et al.,