PS 45-101 - Impact of larval fish abundance on Brandt’s cormorant mortality along the California Coast

Background/Question/Methods

Brandt’s Cormorant (Phalacrocorax penicillatus) populations are highly susceptible to changing environmental conditions. Often known as a “boom or bust” species, their populations variate greatly year to year. One year potentially having half the number of mating pairs and the next, having three times as many. While many species of cormorant can live in vast ranges of temperatures and conditions, Brandt’s is confined to Western North America, namely California and parts of Mexico. Our goal is to find early warning indicators for years that hold detrimental conditions for Brandt’s cormorant numbers and to recognize these detrimental conditions prior to their impacts occurring on the California Current Ecosystem. Using previously collected data from US Fish and Wildlife and California Cooperative Oceanic Fisheries Investigation (CalCOFI) and R statistical software, we quantified changes in prey population of northern anchovy (Engraulis mordax), rockfish (Sebastes, spp.), and sardine (Sardinops sagax). The years 2000 to 2015 were selected as the study period as they contained the most complete stranding and prey data in recent history. We then graphically displayed data and created auto correlation function plots.

Results/Conclusions

Results indicated a positive correlation between the ratio of Brandt’s cormorant strandings to live sightings and northern anchovy larval abundance with a 2-to-4-year lag using auto correlation function plots between shifts in larval abundance of northern anchovy and shifts in the stated ratio. In 2002, there was a rise in the abundance of northern anchovy larvae and in 2005 there was a similar rise seen in the stated ratio. Similarly, 2005 saw the highest peak in northern anchovy larvae in the 15-year period; then, in 2008, the highest peak in the stated ratio in the 15-year period was seen. We concluded that northern anchovies have a maturation period of up to 4 years with maturation occurring most commonly between 2 and 4 years. These data inform us that while larval abundance might not have an immediate impact on Brandt’s cormorant population’s health, the maturation of the larvae into prey for the cormorant over 2-to-4-years may provide insight into Brandt’s cormorant population’s health in advance. Studying how this predator/prey relationship reflects the health of the ecosystem and what is causing the fluctuations in northern anchovy larvae abundance may be key to protecting this vital ecosystem in response to the changing climate.