Bongo Net Retention Rates as Effected by Towing Speed and Mesh Size

Abstract

A comparison of 0.253- and 0.333-mm mesh bongo net retention rates at towing speeds of 1.5 and 3.5 knots showed no significant mesh size related variation in filtration efficiency or in abundance of larval herring at either towing speed. For 1.5-knot tows there was no significant mesh size difference in larval herring length frequency distribution. For 3.5-knot tows, the distributions of lengths were significantly different for the two mesh sizes. The condition of herring larvae was noticeably better in the 1.5-knot tow samples (3%, of yolk-sac larvae caught in 3.5-knot tows had their yolk sac intact compared to 50% from 1.5-knot tows). Feeding incidence was higher for larvae caught in 1.5-knot tows (71%) than for larvae caught in 3.5-knot tows (50%).At a towing speed of 3.5 knots the 0.253-mm mesh zooplankton displacement volumes were significantly greater than the 0.333-mm mesh displacement volumes. At a towing speed of 1.5 knots the difference in displacement volumes was not significant. Adult Oithona spp. and Pseudo-Paracalanus spp., unidentified copepodites, and adult and juvenile Hyperia spp. were caught in significantly greater numbers by the 0.253-mm mesh net at both towing speeds. The 0.253-mm/0.333-mm abundance ratios were greatest for the smaller and more abundant invertebrate taxa. The variation in zooplankton retention rates resulted in large part from differences in mesh size, although catching efficiency was also influenced by tow speed. A substantial proportion of the food items in the guts of all size groups of larval herring consisted of taxa which were not retained by either mesh net (invertebrate eggs, copepod nauplii, Oithona spp., and Pseudo-Paracalanrrs spp. copepodites). Size was the most important factor in prey selection and the average size of food items increased with increased length of herring larvae. On the basis of this analysis, sampling strategies are suggested for future surveys. Key words: bongo nets, retention rates, ichthyoplankton, larval herring, zooplankton, feeding incidence