Summer Zooplankton Abundance and Composition Estimates from 20-m Vertical Hauls in Prince William Sound, Alaska, Using Three Net Meshes

Sturdevant, M

The abundance and composition of mesozooplankton in the top 20 m of the water column were compared between NORPAC nets having 105-, 243- and 303-µm meshes to evaluate net retention and gear suitability for sampling prey fields of age-0 and age-1 forage fish. Single, consecutive vertical hauls were made with each net in daylight at 3 stations in northeastern Prince William Sound, Alaska, on August 5–6, 1995. Individual samples were examined microscopically, and data were pooled by mesh size for analysis. Total zooplankton abundance differed by an order of magnitude between 105- and 303-µm mesh nets (41,000 and 1,400 organisms•m-3). Small calanoid copepods comprised £86% of the composition in all the net mesh sizes. Net retention in the smallest meshes was significantly higher for the small species and stages of calanoids and for several other small taxa (bivalve larvae, invertebrate eggs, barnacle larvae) compared to the largest mesh; conversely, retention for chaetognaths was significantly higher in the largest mesh. The 105-µm mesh net best represented abundances and composition of mesozooplankton consumed by forage fish in summer, predominantly small calanoids, because the others undersampled taxa present in the diets of Pacific herring Clupea pallasi, pink salmon Oncorhynchus gorbuscha, walleye pollock Theragra chalcogramma, Pacific tomcod Microgadus proximus, capelin Mallotus villosus, and Pacific sand lance Ammodytes hexapterus. Although sampling gear bias has long been recognized as critical to estimates of zooplankton production and community structure, this study demonstrates the importance of selecting the appropriate sampling net mesh to characterize prey selection and consumption by planktivorous fish

Summer Zooplankton Abundance and Composition Estimates from 20-m Vertical Hauls in Prince William Sound, Alaska, Using Three Net Meshes

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