Stan Kotwicki

The effect of light intensity on the availability of walleye pollock (Theragra chalcogramma) to bottom trawl and acoustic surveys

Quantitative assessment of semidemersal fish such as walleye pollock (Theragra chalcogramma) is difficult be- cause the proportion of walleye pollock available to standardized surveys varies temporally and spatially. The US National Marine Fisheries Services Alaska Fisheries Science Center conducts bottom trawl (BT) surveys to estimate the demersal portion of the walleye pollock population and acoustic trawl (AT) surveys to estimate the pelagic portion. Both surveys are conducted during daylight hours to minimize variability due to diel changes in vertical distribution. To test if daytime near-bottom light intensity affects the proportion of walleye pollock available to the BT survey, we concurrently measured light and walleye pollock abundance on the Bering Sea shelf. Logistic regression models demonstrated that both light and depth affected walleye pollock abundance estimates by either BT or AT surveys, with more walleye pollock available to the BT survey under high illumination and at shallow depths and less walleye pollock available to the AT survey under these conditions. This finding suggests that daytime survey catchability for walleye pollock depends on depth and light in- tensity and that incorporation of light measurements could improve the precision of abundance estimates of semidemersal species such as walleye pollock. Resume´ : Levaluation quantitative des poissons semi-benthiques tels que les goberges de lAlaska (Theragra chalcog- ramma) est difficile, car la proportion des goberges de lAlaska accessible aux inventaires standardises varie dans le temps et lespace. LAlaska Fisheries Science Center du US National Marine Fisheries Service mene des inventaires au chalut de fond (BT) afin destimer la proportion benthique la population de goberges de lAlaska et des releves acoustiques couples ades inventaires au chalut (AT) pour estimer la proportion pelagique. Les deux inventaires sont realises durant les heures declairement pour minimiser la variabilitedue aux changements de la repartition verticale au cours de la journee. Afin de verifier si lintensitelumineuse pres du fond durant la journee affecte la proportion des goberges de lAlaska accessible a linventaire BT, nous avons mesuresimultanement la lumiere et labondance des goberges de lAlaska sur la plate-forme de la mer de Bering. Des modeles de regression logistique demontrent que la lumiere et la profondeur affectent tous deux lestimation de labondance des goberges de lAlaska, tant dans les inventaires BT que AT: en conditions dillumination elevee et de faible profondeur, un plus grand nombre de goberges de lAlaska est accessible aux inventaires BT et un nombre moins eleveaux inventaires AT. Cette observation fait croire que la capturabilitede la goberge de lAlaska au cours des inventaires de jour depend de la profondeur et le lintensitelumineuse; une incorporation de mesures de la lumi- ere pourrait donc ameliorer la precision des estimations dabondance des especes semi-benthiques, telles que la goberge de lAlaska. (Traduit par la Redaction)

Multispecies acoustic dead-zone correction and bias ratio estimates between acoustic and bottom-trawl data

Multispecies acoustic dead-zone correction and bias ratio estimates between acoustic and bottom-trawl data Kotaro Ono*, Stan Kotwicki, Gjert E. Dingsør, and Espen Johnsen School of Aquatic and Fishery Sciences, University of Washington, PO Box 355020, Seattle, WA 98195-5020, USA Department of Biosciences, Centre for Ecological and Evolutionary Synthesis (CEES), University of Oslo, PO Box 1066 Blindern, NO-0316 Oslo, Norway National Marine Fisheries Service, Alaska Fisheries Science Center, National Oceanic and Atmospheric Administration, 7600 Sand Point Way NE, Seattle, WA 98115, USA Demersal Fish Research Group, Institute of Marine Research, PO Box 1870 Nordnes, NO-5817 Bergen, Norway Marine Ecosystem Acoustic Research Group, Institute of Marine Research, PO Box 1870 Nordnes, NO-5817 Bergen, Norway *Corresponding author: tel: þ47 22 85 44 00; fax: þ47 22 85 40 01; e-mail: kotaro.ono@ibv.uio.no. Present address: The Norwegian Fishing Vessel Owners Association, Slottsgaten 3, NO-5003 Bergen, Norway.