Jack mackerel

Abstract: Using data from stock assesment surveys on pelagic resources during El Nino events of 1972/73, 1982/83, 1997/98, we analyze biological changes on pelagic ecosystems and pelagic fisheries during different stages of development of El Nino phenomenon: emergence, full, final and post-Nino. Results indicate changes in spatial distribution of resources, their concentration and size structure. In anchovy (Engraulis ringens) a decrease in biomass was observed, which was estimated at 1.2 million tons in September 1998, the lowest throughout the 1990s. This resource showed an asymmetric distribution towards the south of Peru. Other pelagic resources increased their biomass during or after Nino events, primarily sardine (Sardinops sagax), jack mackerel (Trachurus murphyi), pacific mackerel (Scomber japonicus), and longnose anchovy (Anchoa nasus). At the end of the El Nino phenomenon we found less productivity but more diversity in the pelagic ecosystem. During the 1997/98 El Nino, the diversity index (Manual de Ecologia, 1a Edition, Editorial Trillas, Mexico, 267pp) increased from 0.87 to 1.23–1.70. In both the emergence stage and fully developed stages of El Nino we found large numbers of sardine and longnose anchovy present simultaneously. Size structure of sardine, jack mackerel, and pacific mackerel showed an increase in juveniles. Anchovy during El Nino showed a single modal group composed of adults, but the post-Nino phase indicated an increase in juveniles with an average length of 6–7 cm. In El Nino conditions spawning among anchovy was low, but among sardine and pacific mackerel it was high. We observed, for the first time during full spawning, juvenile sardines with a total length of 18–20 cm. The anchovy spawning season during the post-Nino phase was considerably lengthened, from April to December 1998. Drastic change occurred in fisheries when monospecific fisheries, based on anchovy before El Nino, became multispecific fisheries based on sardine, jack mackerel, pacific mackerel, longnose anchovy and other species typical of tropical and warm oceans. Landings of anchovy decreased substantially, but those of longnose anchovy, jack mackerel, pacific mackerel, and sardine increased.

Abstract: ABSTRACf Young jack mackerel, Trackurll8 8!f111metril'U8. living offshore are starving while those living nearshore are healthy. These results for sea-caught jack mackerel were determined by using histological and mor· phological criteria that reliably diagnosed the viability of laboratory-raised jack mackereL Both the histological and morphological indices indicated that 350 km offshore about 70% of the first-feeding jack mackerel were starving. In contrast, 12% of the fish collected near islands and banks were starving. In both habitats, mortality rates decreased to zero for jack mackerel at 2 weeks of age. The accuracy of the techniques for prediction of the nutritional state of wild larvae is discussed and evaluated.

Abstract: Field and laboratory experiments were conducted with the purpose of partitioning jack mackerel Trachurus symmetricus larval mortality into portions due to starvation and to predation. Field collections were made to determine larval condition, growth, net retention and production; laboratory experiments were conducted to determine growth and body shrinkage due to preservation treatment. Age-specific starvation and total mortality rates were estimated and predation was inferred as the difference between the two. In the offshore oligotrophic part of the spawning habitat, larvae suffered a high rate of mortality which rapidly declined as they developed. Predation was the major source of mortality of yolk-sac larvae. As the yolks were absorbed and the larvae began to feed, starvation became a significant source of mortality. As the larvae further developed, starvation rapidly declined and predation again became the dominant source of mortality, although at a much lower rate.