Shark Bycatch Research Articles

Shark bycatch in the Japanese high seas squid driftnet fishery in the North Pacific Ocean

From the late 1970s to 1992, large-scale pelagic driftnet fisheries in the North Pacific targeted flying squid ( Ommastrephes bartrami), tunas and billfishes. In the late 1980s, cooperative multinational scientific observer programs began on Japanese, South Korean, and Taiwanese driftnet fleets fishing in international waters. These programs resulted in some of the most comprehensive data on the distribution and abundance of epipelagic species in the North Pacific transition region. Eleven elasmobranch taxa were sighted by observers in the Japanese flying squid driftnet fishery. Blue sharks accounted for 93.7% of the elasmobranch bycatch in 1990 and 1991. In 1991, observers collected biological data on a limited number of shark and fish species, including blue shark ( Prionace glauca) and salmon shark ( Lamna ditropis). Canada experimented with a flying squid driftnet fishery in coastal waters near British Columbia beginning in 1979 but it was terminated in 1987 because of unacceptable levels of bycatch. Blue shark and salmon shark CPUEs were an order of magnitude higher in the coastal Canadian experimental driftnet fishery than in the high seas squid driftnet fishery. The average size of blue sharks in coastal catches was larger than sharks caught in the high seas fishery. The seasonal pattern of CPUE in the Japanese squid driftnet fishery was consistent with the seasonal location of the fleet and the subtropical–subarctic ecological gradient that polarizes salmon and blue sharks. Errors-in-variables regression equations are provided to convert among total length, eye-fork length, and precaudal length measurements.

By-catch of sharks in Patagonian coastal trawl fisheries

In coastal Patagonia, Argentina, an unknown number of species of sharks are frequently caught in bottom-trawl nets. Between 1993 and 1996, 454 trawls by Patagonian coastal fisheries (41˚–51˚S) were analysed; 264 included sharks. Of the seven species of sharks caught, the most frequent was the smoothhound Mustelus schmitti, mainly in Bahía Engaño (off Chubut). The argentine angelshark (Squatina argentina) and the tope (Galeorhinus galeus) were common in the north of Patagonia, the piked dogfish (Squalus acanthias) in the central zone, and the narrowmouthed catshark Schroederichthys bivius in the south. The broadnose sevengill shark (Notorhynchus cepedianus) was rarely caught. A single basking shark (Cetorhinus maximus) was caught in San Matías Gulf (off Río Negro). Although undesirable, the by-catch of sharks is growing as a result of the increase in the fishing activities of the region, and the sustainability of the shark populations in Patagonian coasts is a matter of concern. Resumen. En las costas de Patagonia, Argentina, un número desconocido de especies de tiburones son frecuentemente capturados en las redes de arrastre de fondo. Entre 1993 y 1996 se analizaron a bordo 454 lances de pesca en las pesquerías costeras de Patagonia, entre los 41 y 51˚S, y en 264 de ellos se registraron capturas de tiburones. De las siete especies de tiburones capturadas el mas frecuente fue el gatuzo Mustelus schmitti, principalmente en Bahía Engaño, frente a las costas de Chubut. El pez ángel Squatina argentina y el cazón vitamínico Galeorhinus galeus fueron comunes en el norte de Patagonia, el cazón espinoso Squalus acanthias en la zona central, y el tiburón pintarroja Schroederichthys bivius en el sur. El tiburón gatopardo Notorhynchus cepedianus resultó una especie rara en las capturas. En el Golfo San Matías, frente a las costas de Río Negro, se capturó un ejemplar de tiburón peregrino Cetorhinus maximus. Aunque no deseadas, las capturas de tiburones están aumentando debido al incremento de las actividades pesqueras en la región, y la sustentabilidad de las poblaciones de tiburones en Patagonia es motivo de creciente preocupación. Palabras clave: tiburones, capturas, redes de arrastre, pesquerías costeras, Patagonia.

Blue and Mako Shark by-catch in the Japanese Longline Fishery off South-eastern Australia

During the last 10 years, up to 70 Japanese longline vessels have fished Tasmanian waters of the Australian Fishing Zone each season, targeting bluefin tuna (Thunnus maccoyii). The average seasonal fishing effort in Tasmanian waters is about 3.3 million hooks, and data from observers suggests that, this results in a by-catch of some 34 000 blue sharks (Prionace glauca) each year, representing a weight of about 275 tonnes. In the last few years, fishing effort has increased as the catch rates of southern bluefin tuna have declined. The sharks are discarded after removal of the fins. The actual number of blue sharks hooked is much higher than 34000 because many are released by either shaking or cutting them off the line, although they are often damaged in the process. The majority of blue sharks caught are immature or adolescent females. Smaller numbers of shortfin mako sharks (Isurus oxyrinchus) are also caught and retained both for their fins and their meat. Tasmanian waters represent only one area of the Australian Fishing Zone fished by Japanese longliners.

Age, growth and reproductive biology of the silky shark, Carcharhinus falciformis, and the scalloped hammerhead, Sphyrna lewini, from the northwestern Gulf of Mexico

The silky shark, Carcharhinus falciformis, and scalloped hammerhead, Sphyrna lewini, represent >80% of the shark by-catch of the winter swordfish/tuna longline fishery of the northwestern Gulf of Mexico. This catch represents a potential supplemental fishery, yet little is known of the life histories of the two species. This report relates reproductive biology data to age and growth estimates for 135 C. falciformis and 78 S. lewini. Unlike other regional populations, C. falciformis in the Gulf of Mexico may have a seasonal 12 month gestation period. Males mature at 210–220 cm TL (6–7 yr); females at >225 cm TL (7–9 yr). Application of age at length data for combined sexes produced von Bertalanffy growth model parameter estimates of L∞ = 291 cm TL, K = 0.153, t0 = −2.2 yr. Adult male S. lewini outnumbered adult females in catches because of differences in the distributions of the sexually segregated population. Males mature at 180 cm TL (10 yr); females at 250 cm TL (15 yr). von Bertalanffy parameter estimates for combined sexes of this species were L∞ = 329 cm TL, K = 0.073, to = −2.2 yr.

Predation by Sharks on the Grey Seal (Halichoerus grypus) in Eastern Canada

The increase in population size of the grey seal (Halichoerus grypus) off eastern Canada over the past 20 yr may be attributed to a decrease in shark stocks, their supposed predators. Reduction of the shark population is presumed to have resulted from a directed longline fishery and, of greater significance, from a change in the fishery for swordfish (Xiphias gladius) from selective harpooning to pelagic longlining, which has produced a large bycatch of sharks. The resulting enhanced survival of grey seals is reflected in greater infestation of commercially important fish species by the codworm (Phocanema decipiens).Key words: grey seals, harbour seals, sharks, swordfish, codworm, predation, fisheries