Gulf of California Study finds that the proportions of sardines and anchovies in the diet of pelicans, gulls and terns in spring, can signal the need to reduce fishing efforts in the ensuing season to prevent a fisheries collapse.

Small pelagic fisheries show wide fluctuations, generally attributed to oceanographic anomalies. Most data on these fisheries come from landings, often reporting sustained catches-per-unit-effort (CPUEs) until a decline occurs. It would be useful to be able to predict the decline in advance, and thus avoid it. Research published in Scientific Reports shows significant relationships between fishing effort and seabird diet.

As sardine availability for seabirds declines, CPUEs remain high until the fishery falls, one or two seasons later. A declining proportion of sardines in the seabirds’ diet, combined with the status El Niño, give a reliable forecast of diminishing CPUEs and signals the need to reduce fishing efforts in the ensuing season.

In upwelling regions, such as the California, Humboldt, and Benguela current systems, fishing industries capture the same fish species that seabirds and other marine species feed upon. These small pelagic fishes (known as “forage fish”) constitute around 37% of the world’s commercial landings and often show wide fluctuations, generally attributed to oceanographic-anomalies. Key components of the coastal pelagic marine ecosystem6, their fisheries have been difficult to manage sustainably, and regional economies have been shaken by their collapses. Forage fish transfer energy from microscopic plankton to larger predators and constitute a fundamental food source for marine mammals, seabirds and larger fish species, many of which are also commercially important.

In the Gulf of California (also known as the Sea of Cortez) there are no robust fishery-independent stock assessments available for small pelagic fisheries. Seabirds, however, face similar challenges as does the fishing fleet to find available prey and harvest it.

The relationship between the proportion of sardine in the seabird diets and sardine CPUE shows that sardine catchability decreases faster for seabirds than it does for the fleet, which can maintain high CPUEs even when seabirds have shifted entirely away in search of other fish species, mostly anchovies. Thus, even when CPUEs are high and there is no evidence of decreasing catchability in the fishery itself, other ecosystem components, such as seabirds, may be confronting a decline in available sardine. Availability may further decline during El Niño anomalies, when a deeper thermocline may prevent seabirds from reaching their prey. Thus, the El Niño anomaly and the relative abundance of sardine in the seabirds’ diets can jointly predict the sardine CPUE by the commercial fleet.

Elegant Terns and Heermann’s Gulls feed on juvenile pelagic fish, while Brown Pelicans feed mostly on the same adult fish that are taken by the fleet. This very likely explains why tern and gull diets are statistically more related to CPUEs in the following fishing season, while Brown Pelican diets are more related to CPUEs during the same season.

Sardine catchability is much higher for the fleet than it is for seabirds (and possibly for other ecosystem components). The increased sensitivity of the seabird diets relative to CPUE suggests that the seabirds are more diversified in their prey base and able to switch to other prey species when sardine availability starts to decline. Observed reductions in the proportion of sardines in seabirds’ diets in the Gulf of California Midriff before the beginning of the fishing season may provide a useful way of monitoring the fishery and forecasting the success of the fishing fleet.

Map credit: Map of the Midriff Islands, Gulf of California, Mexico, indicating the location of Isla Rasa and other islands (drawn by Cathy Moser Marlett)

Further Reading:
Enriqueta Velarde, Exequiel Ezcurra &, Daniel W. Anderson, Seabird diets provide early warning of sardine fishery declines in the Gulf of California. Scientific Reports 3, Article number: 1332 doi:10.1038/srep01332 (2013).