Precision of larval herring abundance and body length assessments when sampling with BONGO nets for spawning site mapping

Abstract

Data on the abundance and size structure of larval and juvenile fish are commonly used in environmental research. Knowledge about the repeatability of the results of a given sampling tool is an important part of planning sample collection schedules. The aim of this research was to evaluate the variance in the abundance and length of herring larvae (6.9–24.4 mm SL) collected with 505 bongo nets during 12 cruises conducted during April-June at 6–9 stations in the Pomeranian Bay, Baltic Sea, from 2015 to 2018. The differences (mean value of % deviation from the mean) between the left and right bongo nets (compared during 17 tows) were 21.6% for larval abundance and 3.6% for the SL measurement. When three tows were subsequently conducted at the same station (compared at 6 stations), the differences were 19.7% and 10.1% for abundance and the SL assessment, respectively. When samples were collected at the same stations on consecutive days (compared for 75 paired days), the error was 33.4% for abundance and 6.1% for SL. Several factors (filtered-water volume, larval abundance, larval size, wind strength, and station location) were tested for their effect on the magnitude of the variations, but only sampled larvae abundance was found to be significant with lower precision for low abundance. Although the above low precision level did not significantly affect the overall assessment of larval abundance and SL within the investigated geographical area within a given year, it should be considered that differences of ca. ± 10% (for SL) and ca. ± 33% (for abundance) are within the sampling method errors, which correspond to ca. thirteen percent (for SL) and 200% (for abundance) of the absolute differences expressed as % of the lower value. While the present results were obtained for herring, we believe that the conclusions are also applicable to early life stages of other fish species, especially clupeids such as sprat, menhaden, and smelt.