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The Impact of Costa Rica’s Longline Fishery on Silky Sharks

The Impact of Costa Rica’s Longline Fishery on Silky Sharks

Longlining is one of the major fishing industries in Costa Rica, having been introduced by the Taiwanese in the early 1980s. But the rapid growth of this high bycatch fishery has had severe impacts on marine wildlife, including silky sharks. Change is needed to protect sharks as well as stocks of fish and endangered species such as sea turtles.

The Rapid Growth of the Longline Fishery in Costa Rica

Longline fishing initiated in Costa Rica in 1982, when the Cooperation Agency of Taiwan sent an official diplomatic mission to teach Costa Rica how to fish with longlines and initiate an industry. This effort was largely motivated by declining stocks of coastal fish in Costa Rica, and that the vast wild ocean areas within Exclusive Economic Zone that had not been fished.

The industry boomed as a result of major investment by the Taiwanese. By 1988, Costa Rica had the largest longline fleet of Latin America.

Currently, Costa Rica boasts a fleet of 300 “medium scale” longliners (iced holds, two week autonomy, 10-20 ton capacity), and 100 “advanced scale” longliners (freezer capacity, 6 month autonomy, 100-200 ton capacity).

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The Severe Impacts on Silky Sharks

The growth in longlining has contributed to a decline in the population of silky sharks.

Silky sharks represented 70 percent of Costa Rica’s shark catch by the domestic and foreign flagged longline fishery from 2003 to 2008. During this same time period, silky shark landings declined from 5,657 tons to 3,690 tons.

Silky shark catch rates have declined from 4.7 individuals per 1000 hooks set in 1999 to less than 1 per 1000 hooks set in 2008.

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More than 90% of all silky sharks landed in 2010 were undersized (below the size of first maturity).

In 2003, Costa Ricans consumed 4,500 tons of shark meat per year. Currently consumption is down to 1,500 tons.

A recent increase in the Eastern Tropical Pacific silky shark catch index is not the result of greater abundance nor recovering stocks, but rather a shift of availability due to strong El Niño conditions. This “false positive” trend is not strong enough to dismiss the urgent need for precautionary management actions.

The Decline of Mahi Mahi

Longlining has also contributed to lower catch rates for mahi mahi.

Mahi mahi catches have declined from 50 individuals per 1000 hooks in 1999, to 16 individuals per 1000 hooks in 2008, indicating that mahi mahi is also overfished.

Mahi mahi is a highly seasonal fishery resource, with peaks of abundance from December to May. From June to November mahi mahi is scarce and practically absent from catches.

The Interaction of the Silky Shark and Mahi Mahi Fisheries

Highest silky shark catch rates occur when mahi mahi catches are lowest, particularly from August to October. During these months, silky sharks constitute up to 85% of all species landed, which mean this is not a mahi mahi fishery but is instead a targeted silky shark longline fishery.

Recommendations to Protect Silky Sharks

  • A three-month closure of the longline from August to October would save an array of endangered marine species, from sharks, to turtles, to billfish.
  • A permanent ban on steel leaders in the longline fishery would save sharks year round.
  • Fishing effort in general needs to be reduced.
  • Shark meat should not be eaten.