Why are California sea lion pups starving? ‘Any fishing on sardines right now is overfishing, as the population is not even replacing itself, much less providing a surplus’

By Ben Enticknap
23 February 2015 (Oceana.org) – Recent national news stories have shown emaciated California sea lion pups being rescued and admitted to marine mammal rehabilitation centers. The sight of hundreds of withered sea lions is reminiscent of the all-too-recent California sea lion “unusual mortality event” in 2013 followed by a significant number of stranded and dead sea lions last year. Dying sea lions and strandings are becoming less unusual and a more frequent occurrence. In fact, the 2015 sea lion deaths are already on pace to exceed the 2013 numbers. About 70 percent of sea lion pups are expected to die this year before weaning age. So, why are these pups stranding and dying? It is simple. The sea lions are starving. Nursing female sea lions don’t have enough food to eat; primarily fatty forage fish like sardine and anchovies and they are spending more time foraging, which means less time feeding their pups. In turn, the pups are not getting the nutrition they need, and people are finding them stranding on beaches, weak, emaciated and dying. As the National Oceanic and Atmospheric Administration (NOAA) stated following the 2013 mortality event, these sea lion pups are starving and malnourished due to a “change in the availability of sea lion prey, especially sardines” (NOAA Fisheries 2014). This preliminary finding suggested the sardine population was spawning further offshore, outside of the foraging range of nursing California sea lions. At the same time, however, other key forage fish prey like anchovy and hake are spawning closer to shore or spawning locations are unchanged. So, a shift in the spawning location of sardine alone does not explain the cause. What is happening is that there is an overall decline in the abundance of forage species. Importantly, NOAA scientists are now corroborating the hypotheses that seas lions are starving due to a lack of prey, not just that the prey moved someplace else. At a scientific conference in December 2014, NOAA scientists reported

In the last decade, key components of sea lion diet have either almost disappeared (anchovy), or decreased in biomass (sardine), while market squid abundance increased…Increasing squid abundance and declining sardine and anchovy abundance, reduced the caloric value of available forage in the last 10 years… Occurrence of numerous malnourished pups may now become frequent as the sea lion population experiences food limitation (McClatchie, et al., 2014).

Indeed the sea lion population has been steadily increasing at a rate of over five percent per year and the latest population estimate is that there are over 300,000 California sea lions off the U.S. West Coast (NOAA Fisheries 2015). Nobody knows what the sea lion carrying capacity is (how many sea lions the environment can naturally support), but right now, the population is clearly being limited by the abundance and availability of forage fish. The one thing we can address is the commercial fisheries that are competing with the sea lions and other ocean wildlife by targeting sardine and anchovy. In 2010 two NOAA scientists (Zwolinski and Demer 2012) published a paper predicting the collapse of the Pacific sardine population. The authors concluded in the abstract:

[a]larming is the repetition of the fishery’s response to a declining sardine stock – progressively higher exploitation rates targeting the oldest, largest, and most fecund fish.

Though the paper was subject to a great to deal of controversy and debate within the agency, the scientists’ predictions came true. The Pacific sardine population has collapsed. The 2014 sardine assessment found that the “spawning biomass” is at the lowest levels in over 15 years (Hill, et al., 2014). The current population assessment estimates the spawning biomass at 306,237 metric tons, which is far below the “critical biomass threshold” (or the minimum amount of fish that should be in the ocean before fishing should occur) of 740,000 metric tons identified in the Zwolinski & Demer 2012 study. Since 2007, the sardine population has declined by 74 percent (1.05 million metric tons) and it continues to decline with no clear signs of recovery (Hill, et al., 2014). Over the most recent seven year period the fishery has removed 715,000 metric tons of sardines. Therefore, while we can only speculate what the decline would have been in the absence of fishing, over two thirds (68 percent) of the recent seven-year decline are attributable directly to fishery removals. In other words, any fishing on sardines right now is overfishing, as the population is not even replacing itself, much less providing a surplus. [more]