Contrasting years of prey levels, feeding conditions and mortality of larval walleye pollock Theragra chalcogramma in the western Gulf of Alaska

Abstract

Walleye pollock in Shelikof Strait, Gulf of Alaska, spawn in a n area with strong interannual differences in the oceanic environment. Feeding conditions and mortality of walleye pollock larvae in Shelikof Strait were compared in 2 consecutive years of markedly contrasting oceanographic conditions. In 1990, winds were relatively calm, and a large eddy was formed in the lower portion of the strait; walleye pollock larvae were found concentrated in this eddy feature. In 1991, winds were very strong and sea surface temperatures were anon~alously cold Flow through the Shelikof region was strong in that year, and larvae were sparse. In 1990, copepod naupliar abundance was high throughout the study area. There were no geographic differences in feeding intensity of larvae, RNA content or larval growth. In 1991 the major differences occurred between ~n s h o r e and offshore stations. Con~paring conditions in 1990 and 1991, naupliar abundance, larval feeding intensity, RNA content and length-atage were all low in the stormy conditions of 1991 In 1991 estimated mortality was significantly higher than that measured in 1990, although part of the loss could have been due to strong advection out of the area. Survival of expatriated larvae is discussed in light of very low juvenile abundances in 1991. This study shows the dramatic effect of environmental conditions on early larval survival rates.

Estimation of starvation and diel variation of the RNA/DNA ratios in field-caught Sardina pilchardus larvae off the north of Spain

“…Moreover, in the present study, zooplankton biomass seems to explain part of the variation in RNA/DNA ratios which was revealed by the significant correlation between this index and the potential prey of sardine larvae. This seems to agree with several other recent studies, namely, Ca.nino et al (1992), Canino (1994), Bailey et al (1995), Su.thers (1996, and Suthers et al (1996). However, results of sardine larval condibon measured by histological methods (McFadzen et al 1997), on the same cruises, indicated the lack of any overall correlation between food availability and the RNA/DNA index.…”

“…RNA/ DNA ratios have been used to assess the nutritional condition of a wide range of manne organisms, predominantly fish larvae (Buckley 1984, Roblnson & Ware 1988, Suthers 1992, Bailey et al 1995. Thls index is based on the assumption that the amount of deoxyrlbonucleic acid (DNA), the pnmary carrier of genetic information, is stable under changlng environmental situations, while the amount of ribonucleic acid (RNA), which is directly involved in p r o t e~n synthesis, is affected by the nutritional condition of the organism.…”

Climate change and control of the southeastern Bering Sea pelagic ecosystem

“…To show trends, the scales of the y-axis vary between strata. Bailey et al, 1995Bailey et al, , 1996, perhaps in part because of greater abundance of zooplankton. Ice and cold temperatures significantly delay hatching of pollock eggs, which may result in higher total egg mortality (Blood, 2002), and causes a mismatch between the peak abundance of larval pollock and their prey .…”

Planktonic larval mortality rates are lower than widely expected

“…The first approach is the so-called horizontal life table method, which parallels the traditional method of estimating mortality in terrestrial populations: a time series of abundances of stages are analyzed to determine the proportion of individuals that survive from one observation to the next (Wood 1994). This approach can accommodate temporal variation in demographic parameters (including the mortality rate) but is very sensitive to advection; that is, it is impossible to distinguish losses due to mortality from losses due to the advection of larvae away from the sampling station (Bailey et al 1995, Aksnes et al 1997. Thus, this method is appropriate for populations in enclosed bodies of water or in shelf regions with minimal advection (Ohman et al 2004) but is generally inappropriate for larvae, the abundance of which can be strongly influenced by advection (Caley et al 1996).…”