Size-based hydroacoustic measures of within-season fish abundance in a boreal freshwater ecosystem.

Riley A Pollom,George A Rose,Z Daniel Deng

doi:10.1371/journal.pone.0124799

Abstract

Eleven sequential size-based hydroacoustic surveys conducted with a 200 kHz split-beam transducer during the summers of 2011 and 2012 were used to quantify seasonal declines in fish abundance in a boreal reservoir in Manitoba, Canada. Fish densities were sufficiently low to enable single target resolution and tracking. Target strengths converted to log2-based size-classes indicated that smaller fish were consistently more abundant than larger fish by a factor of approximately 3 for each halving of length. For all size classes, in both years, abundance (natural log) declined linearly over the summer at rates that varied from -0.067.day-1 for the smallest fish to -0.016.day-1 for the largest (R2 = 0.24–0.97). Inter-annual comparisons of size-based abundance suggested that for larger fish (>16 cm), mean winter decline rates were an order of magnitude lower (-0.001.day-1) and overall survival higher (71%) than in the main summer fishing season (mean loss rate -0.038.day-1; survival 33%). We conclude that size-based acoustic survey methods have the potential to assess within-season fish abundance dynamics, and may prove useful in long-term monitoring of productivity and hence management of boreal aquatic ecosystems.

Highlights

In freshwater ecosystems, the relative abundance and dynamics of size-classes of organisms are keys to understanding energetics and production [1]
The acoustically-derived fish counts surveyed at Lac du Bonnet during the summers of 2011 and 2012 indicated that TS distributions ranged from approximately -52.6 dB to a very few tracks that measured in the low -20 dB range (Fig 2)
Day of year explained between 74.7 and 97.4% of variation in seasonal abundance declines for the 5 size classes with the exception of the largest size class in 2012 which declined over the season but with greater variability among surveys (Table 3)

Summary

Introduction

The relative abundance and dynamics of size-classes of organisms are keys to understanding energetics and production [1]. In these ecosystems, predators and prey often display distinct and predictable body size ratios [2] that may reflect trophic levels [3], [4]. A positive relationship between body size and trophic level is likely in aquatic systems because fish are morphologically constrained by gape limitation, and limited to prey within a specific size range [5], [6]. PLOS ONE | DOI:10.1371/journal.pone.0124799 April 15, 2015

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