Analysis Shows Effects of Exploitation on Species and Size Composition of Pelagic Fish Community

A decline in the percentage of large fish in the catch of Hawaii-based deep-set longline vessels was accompanied by an increase in 
               the percentage of fish discarded.
A decline in the percentage of large fish in the catch of Hawaii-based deep-set longline vessels was accompanied by an increase in the percentage of fish discarded.

Recent research by Jeffrey Polovina and Phoebe Woodworth-Jefcoats of the PIFSC Ecosystems and Oceanography Division indicates that fishing is likely the cause of observed changes in the species and size composition of the fishery's catch. Moreover, the fishery's effects extend to fishes smaller than those exploited by the fishery.

Polovina and Woodworth-Jefcoats examined trends in the species and size composition of the Hawaii-based deep-set longline fishery catch and the causes for these trends. By examining 16 years (1996 - 2011) of catch and effort records for 23 species, they found a 25% decline in the proportion of large fish (≥ 15 kg) in the catch and a concurrent increase in the catch of noncommercial species that are subsequently discarded. Ecosystem modeling indicates that these changes are likely fishery-induced and extend to fishes smaller than those exploited by the fishery.

As longline fishing effort (number of hooks fished) increased from 1996 to 2011, the annual catch of large fish declined by nearly 50% from 1996 to 2011 while the catch of small fish increased by about 25%. Combined, these trends led to a decline in total catch biomass during the period (Fig. 9). Further investigation with a size-based numerical food web model found that as fishing mortality increases, large fish abundance declines by up to 50% while small fish abundance increases by about 15%. Additionally, the abundance of fish smaller than those exploited by the fishery (as small as 0.1 kg) declines.

Until about 2005, the total annual catch biomass (all species caught) in the deep-set Hawaii longline fishery grew with increased 
               fishing effort. Since then it has declined at sustained high levels of effort. For catch biomass, both observed values and the 
               fitted model are shown.
Until about 2005, the total annual catch biomass (all species caught) in the deep-set Hawaii longline fishery grew with increased fishing effort. Since then it has declined at sustained high levels of effort. For catch biomass, both observed values and the fitted model are shown.

Taken as a whole, the observed and modeled trends indicate that size-based predation plays a key role in structuring the subtropical Pacific ecosystem. As the largest fish species (including target species such as bigeye tuna) are exploited by the fishery their declining population exerts less predation pressure on smaller fish, thus allowing the populations of smaller (often less commercially valuable) fish to grow. The size-based model results also show that catch-based indicators underestimate the full ecosystem impact of fishing.

In addition to changes in the size composition of the observed catch, a change also occurred in the species composition of the catch. The catch of smaller, non-commercial species such as lancetfish and snake mackerel increased from 30% to 40% of the total catch. Numerically, lancetfish catch has surpassed bigeye tuna catch and this non-commercial species is now the most frequently caught fish in the Hawaii-based deep-set longline fishery.

An increase in the abundance of smaller species and a decline in the abundance of larger species may increase the vulnerability of the ecosystem if the smaller species have faster turnover, shorter life spans, and track environmental change more closely.