William H. Bayliff

A Markov movement model of yellowfin tuna in the eastern Pacific ocean and some analyses for international management

Abstract A movement model is developed to quantify the probabilities of movement for yellowfin tuna in the Eastern Pacific Ocean. The model specifies a probability transition matrix for the odds of a given-aged individual moving from one 5° 2 spatial stratum to an adjacent spatial stratum during each month. A novel feature of the model is the description of discrete movement with three parameters (velocity, diffusion, and direction) in a manner analogous to continuous movement models. A maximum likelihood approach is taken to estimate movement and catchability parameters for data obtained from tagging experiments. Yield-per-recruit analysis is a critical, though often neglected, tool required to evaluate the need for international management of tunas. A modified mean residence time calculation is shown which addresses some consequences of eliminating the harvest of smaller sub-optimal sized yellowfin tuna.

The Fisheries for Tunas in the Eastern Pacific Ocean

Abstract The principal species of tunas taken by the fisheries of the eastern Pacific Ocean are yellowfin, Thunnus albacares, bigeye, T. obesus, skipjack, Katsuwonus pelamis, Pacific bluefin, Thunnus orientalis, and albacore, T. alalunga. The first three are tropical species, while the other two inhabit temperate waters. The greatest catches of tunas are taken by purse seines—large nets that encircle entire schools of fish. Yellowfin, bigeye, skipjack, and bluefin are caught mainly by purse seines. Longlines consist of a mainline—about 60 nautical miles long if deployed in the open ocean—and branch lines, each with a baited hook at the end of it. Longlines catch yellowfin, bigeye, bluefin, and albacore. Trolling is conducted from a relatively small vessel that tows lines, each with an artificial lure at the end of it, through the water. Trolling vessels catch mainly albacore. Nearly all the purse seiners and trollers that fish in the eastern Pacific Ocean are registered in nations of the western hemisphere, while nearly all of the longline vessels are registered in Far Eastern nations. Tuna vessels stay at sea for weeks or months at a time, so their catches must be frozen at sea. The catches of longline vessels are nearly always sold fresh, so they must be handled with great care and frozen at very low temperatures. Purse-seine vessels catch large amounts of fish—sometimes 100 or more tons in a single day—so it is not often feasible to handle the fish as carefully as longline-caught fish. These fish are nearly always canned. In the eastern Pacific Ocean, the catches of yellowfin and skipjack are greatest, followed by those of bigeye, albacore, and bluefin, in that order. The Inter-American Tropical Tuna Commission, an international organization with a membership of 21 countries, performs research on tunas and other animals that associate with them, and makes recommendation for regulation of the fisheries, if necessary. If and when regulations are adopted, it is the responsibility of each member to enforce the regulations on its vessels.