Climate Variability & Marine Fisheries

A change occurred in the late 1980's in the ocean conditions along the Northwestern Hawaiian Islands (NWHI), the 15,000 km chain of islands reefs and atolls that stretch northwest of the main Hawaiian Islands. The effects were seen at several trophic levels, from seabirds and monk seals to reef fishes and spiny lobsters. Survival of red-footed booby and red-tailed tropicbird eggs and chicks decreased by about 50% between the early and late 1980s; this was likely the result of extended time away from the nests by parents searching for scarce food resources (oceanic flying fish, squid). Monk seal pups were emaciated and their survival dropped in the late 1980s and early 1990s, probably due to lack of food in the waters around the reefs; this is supported by reef fish densities, which dropped by about one third by the early 1990s. The spiny lobster fishery collapsed in 1990, with the catch rate dropping by 40% in one year.

Northwestern Hawaiian Islands (NWHI) Click on image to enlarge

It is often difficult to separate the effects of climate changes from the effects of fishing, but the NWHI offers a variety of small island reef complexes, with some unfished areas for comparison. Drops in densities of reef fish occurred in unfished areas. Sea birds and monk seal pups are not subject to fishing; their declines appear to reflect declines in their food supply, which also is not subject to fishing pressure. The declines in spiny lobster were fastest on one of the harvested reefs, but declines were also observed in an unharvested refuge.

Time Series of Indices of Productivity Click on image to enlarge

These changes in the productivity of the area around the NWHI appear to be tied to a shift in ocean conditions around 1989. The Aleutian Low Pressure System was more intense and located more to the south from 1977-88. As conditions changed in the mid-1980s the winter storm winds weakened, resulting in lower vertical mixing, fewer nutrients in the photic zone, and thus reduced productivity in the open ocean. It is important for fisheries managers to be aware of changes in ocean productivity. Conditions are not always be optimal, and harvest policies developed under one productivity regime may not carry over to the next. This type of environmental variability and its ecosystem-wide effects, must be considered when managing such complex systems.

TEXT REFERENCES:

1)	Polovina, Jeffrey J., and Wayne R. Haight. 1999. Climate variation, ecosystem dynamics, and fisheries management in the Northwestern
	Hawaiian Islands In: Ecosystem Approaches for Fisheries Management. Alaska College Sea Grant Pub. AK-SG-99-01, pp. 23-32.
2)	Polovina, J. J., G. T. Mitchum, N. E. Graham, M. P. Craig, E. E. DeMartini, and E. N. Flint. 1994. Physical and Biological Consequences
	of a Climate Event in the Central North Pacific. Fisheries Oceanography 3: 15-21.
3)	Trenberth, K. E., and J. W. Hurrell. 1994. Decadal Atmosphere-ocean Variations in the Pacific. Climate Dynamics. 9:303-319.