Thomas K. Wilderbuer

Analysis of Fishing Power Correction Factor Estimates from a Trawl Comparison Experiment

Abstract Four published analytical techniques were applied to comparative trawl data to obtain fishing power correction (FPC) factors for 12 major commercial species that were caught by two resource assessment trawls used by the National Marine Fisheries Service (Alaska Fisheries Science Center). Fishing power correction techniques included ratios of catch per unit effort (CPUE), randomized block analysis of variance (ANOVA), standard least-squares regression, and a method developed by Kappenman. All four estimators generally gave similar results for species that were clearly caught in higher proportion by one trawl, but they differed in the magnitude of the adjustment. For Pacific cod Gadus macrocephalus and sablefish Anoplopoma fimbria, which were caught at very similar rates during the experiment, FPC estimates close to 1.0 (no difference between trawls) were obtained from each technique, but they differed regarding which trawl was most efficient. The Kappenman estimator gave FPC estimates of 1.0 for s...

Sex-Specific Natural Mortality of Arrowtooth Flounder in Alaska: Implications of a Skewed Sex Ratio on Exploitation and Management

Abstract Conservative fisheries management in Alaskan waters typically limits harvest to maintain a target biomass of female spawners (40% of the female biomass observed in the absence of fishing). This harvest calculation does not consider the male population level for a species. If natural mortality rates (M) differ between sexes and if females outlive males, then a reduction in males relative to females could diminish the reproductive potential of the stock. This problem was examined in the present study by estimating separate sex-specific M-values for the arrowtooth flounder Atheresthes stomias in Alaska, where females are almost always found at higher proportions than males during trawl surveys. Age data indicated that females outlive males, and estimates of M ranged from 0.10 to 0.33 for females and from 0.16 to 0.51 for males. Based on four estimation methods, male M was consistently higher than female M. Simulated harvest scenarios in which male M and selectivity were varied indicated that increas...

The role of flatfishes in the organization and structure of the eastern Bering Sea ecosystem

We evaluated the role of flatfishes in the organization and structure of the eastern Bering Sea ecosystem using the Ecopath/Ecosim approach. As basic input data for the Ecopath/Ecosim model, we used estimates of biomass from bottom trawl surveys and age-structured population models, production/biomass (P/B) ratio, consumption/biomass (Q/B) ratio, diet composition (DC), and fisheries harvests for each component of species or species groups. We estimated the trophic level of each component, niche overlaps among flatfishes, and the impacts of competition and predation on flatfish species in the eastern Bering Sea ecosystem. Based on those estimates, we developed the tropho-dynamic structure of the ecosystem, and the model was used to simulate ecological effects of fishery exploitation patterns. No single flatfish species appeared to have a profound and uniquely important role in the organization and structure of the ecosystem. Instead, the most important component among the guild of flatfish species appeared to be yellowfin sole Pleuronectes asper, which had greater biomass than other flatfish and a relatively diverse diet among the small flatfish species. Pacific halibut Hippoglossus stenolepis, Greenland turbot Reinhardtius hippoglossoides, and arrowtooth flounder Atheresthes stomias were important keystone predators in the eastern Bering Sea ecosystem together with some groups of marine mammals and sea birds. Intra flatfish complex cannibalism was not observed, however, substantial diet overlaps were common in the flatfish guild system.

Erratum to: The role of flatfishes in the organization and structure of the eastern Bering Sea ecosystem

In the 5th paragraph of the ‘‘Model structure’’ subheading of the ‘‘Data and methods’’, lines 6–8 of the text should read: ‘‘...the immigration of i, Yi is the yield of i (i.e., its catch in weight, with Yi = FiBi, where F is the fishing mortality rate), Bj is the biomass of the consumers or predators,...’’. On the 2nd and 4th pages of Table 2, ‘‘35 Myc. & Bathy.’’ in the column headed ‘‘Prey/predator’’ should be ‘‘35 Myc. & bathy.’’ In Fig. 4, ‘‘Myc. & Bathy.’’ on the y axis should also be ‘‘Myc. & bathy.’’ In addition, under the ‘‘Ecosim analyses’’ subheading of the ‘‘Results’’, in the 1st paragraph lines 9–11 should read: ‘‘...Arrowtooth flounder had biomass 3.7 times higher in 2005, and Greenland turbot showed the fastest rate of decrease’’. In the 2nd paragraph lines 17–18 should read: ‘‘...It was only the rate of increase or decrease in the biomass of flatfishes...’’.