Application of in situ target-strength estimations in lakes: examples from rainbow-smelt surveys in Lakes Erie and Champlain

Abstract

Acoustic abundance of fish depends directly on the target strength (TS) of the fish surveyed. We analyzed 70 and 120 kHz acoustic data from two lakes with abundant rainbow-smelt (Osmerus mordax) populations. Using repeated surveys through the summer growing season, we derived a relationship between TS (dB) and fish length L (cm) at 120 kHz (TS = 19.9 log10 L − 67.8). Values for 70 kHz were similar. In situ TS increased with fish density, indicating a bias from accepting multiple targets at high fish densities. Correcting for this bias increased estimates of smelt abundance by up to 18% in Lake Erie and up to 100% in Lake Champlain. Multiple modes in the TS distributions observed for older fish do not reflect different size groups, as the same modes can be observed from measurements from a single fish. Smelt released gas bubbles during the evening ascent, and these bubbles had TS (−60 to −58 dB) within the range of TS observed from the fish. Gas-bubble release occurred mostly during the migration. Conducting surveys after the ascent is completed will decrease bias associated with counting bubbles as fish.

Hydroacoustic Estimates of Abundance and Spatial Distribution of Pelagic Prey Fishes in Western Lake Superior

“…These relationships were established for species encountered that corresponded in size to those collected in our trawl samples. These equations tend to overestimate length of small targets and underestimate lengths of large targets (Rudstam et al 2003). We recommend using alternative equations if the size range of fish being sampled falls outside the size range used to develop the relationship.…”

Status of pelagic prey fishes in Lake Michigan, 2013

“…Mean mass of bloater in these cells was estimated using the mass-TS equation of Fleischer et al (1997). If mean TS was ≤ -45 dB, we assumed the fish were large rainbow smelt and estimated mean mass from mean length, predicted using a TS-length equation (Rudstam et al 2003).…”

“…The initial three strata were retained, but their size was modified based on data collected in 2003 as well as NOAA Coast Watch Great Lakes node maps of sea surface temperature from 2001-2003. In 2007, the number of transects in each stratum was optimized based on stratum area and standard deviation of biomass using methods in Adams et al (2006).…”

“…Based on a target strength (TS) -length relationship for alewives (Warner et al 2002), the applied lower threshold should have allowed detection of our smallest targets of interest (≈20 -30 mm age-0 alewife). This threshold may have resulted in underestimation of rainbow smelt density because TS of age-0 rainbow smelt in August is as low as -74 dB (Rudstam et al 2003).In order to assign species and size composition to acoustic data, we used a technique described by Warner et al (2009), with different approaches depending on the vertical position in the water column. For cells with depth < 40 m, midwater trawl and acoustic data were matched according to transect, depth layer (0-10, 10-20 m, etc., depending on headrope depth and upper depth of the acoustic cell), and by bottom depth.…”

“…Mean mass of bloater in these cells was estimated using the mass-TS equation of Fleischer et al (1997). If mean TS was ≤ -45 dB, we assumed the fish were large rainbow smelt and estimated mean mass from mean length, predicted using a TS-length equation (Rudstam et al 2003).As recommended by the Great Lakes acoustic SOP Rudstam et al 2009), we used a number of techniques to assess or improve acoustic data quality. We used the N v index of Sawada et al (1993) to determine if conditions in each acoustic analysis cell were suitable for estimation of in situ TS.…”