Pelagic fish assessments often combine large amounts of acoustic-based fish density data and limited midwater trawl information to estimate species-specific biomass density. We compared the accuracy of five apportionment methods for estimating pelagic fish biomass density using simulated communities with known fish numbers that mimic Lakes Superior, Michigan, and Ontario, representing a range of fish community complexities. Across all apportionment methods, the error in the estimated biomass generally declined with increasing effort, but methods that accounted for community composition changes with water column depth performed best. Correlations between trawl catch and the true species composition were highest when more fish were caught, highlighting the benefits of targeted trawling in locations of high fish density. Pelagic fish surveys should incorporate geographic and water column depth stratification in the survey design, use apportionment methods that account for species-specific depth differences, target midwater trawling effort in areas of high fish density, and include at least 15 midwater trawls. With relatively basic biological information, simulations of fish communities and sampling programs can optimize effort allocation and reduce error in biomass estimates.
The USGS Great Lakes Science Center conducted acoustic/midwater trawl surveys of Lake Huron in 1997, 2004, and 2005. The 2005 survey was conducted during SeptemberOctober, and included transects in Lake Huron’s main basin, Georgian Bay, and the North Channel. Estimates of pelagic fish density were higher in 2005 than 2004, primarily due to increased rainbow smelt abundance in the North Channel and a lakewide increase in age-0 bloater abundance. However, biomass estimates did not differ significantly between the two years, and were lower than estimates from the initial survey in 1997. Alewife density and biomass remained low; alewife density in 2005 was about 1 fish per hectare, compared with a density of 580 fish per hectare in 1997. Slight increases in the density of sticklebacks and emerald shiners during 2005 did not contribute substantially to pelagic fish biomass. Between 1997 and 2004-05, Lake Huron’s pelagic fish biomass decreased from about 150 kg per hectare to 10-20 kg hectare. This was due to almost complete disappearance of alewife and a change in age and size structure in the bloater population from dominance by adults to increased prevalence of smaller or younger bloaters. Rainbow smelt density varied among regions and North Channel density and biomass may have been higher than overall lake-wide densities observed during 1997. Lake Huron appears to have lost a substantial amount of pelagic fish biomass between 1997 and 2004-05. Results of the acoustic survey support observations of lower prey abundance observed in bottom trawl surveys, but suggest that overall prey availability remains higher in Georgian Bay and the North Channel than in the main basin.
Acoustic surveys of fish are a foundational component of many fisheries monitoring programs, including surveys in the Great Lakes. These surveys are conducted with traditional crewed and motorized vessels, but fish avoidance of these types of platforms has been reported in multiple studies, potentially biasing estimates. A quiet uncrewed surface vessel, Saildrone, was equipped with a 120 kHz Simrad EK80 transducer and deployed in Lakes Huron, Michigan, and Superior in the summers of 2021 and 2022. The drone was then overtaken by numerous motorized vessels using transducers with the same frequency. The average target depth, target strength, and nautical area scattering during overtakes were compared. We looked for a fish behavioral response with General Additive Models using distance from the Saildrone to the vessel as the predictor. We also compared the effectiveness of acoustic surveys from each platform with analyses of variances over 2 km sections of the overtake. Fish showed a limited response to approaching vessels, and acoustic estimates of fisheries relevant measures were similar between Saildrone and motorized vessels. Findings from this work will inform interpretation of acoustic data in the Great Lakes and provide the largest scale testing of fish avoidance to acoustic surveys to date.
Abstract To predict effects of modifying the daily bag limit (DBL) on management of Chinook salmon Oncorhynchus tshawytscha and alewives Alosa pseudoharengus , we analyzed harvest and effort data collected from both charter and noncharter anglers during 1997–2005 in Michigan waters of Lake Michigan. Overall, the percent of anglers who caught the DBL of three Chinook salmon was low for charter (10.2%) and noncharter (3.3%) angler groups. However, during 1997–2005, the percent of anglers that caught their DBL increased from 3.3% to 26.5% for charter anglers and from 0.5% to 7.8% for noncharter anglers. To predict harvest under a higher DBL, we fit a discrete negative binomial distribution to the observed daily harvest distribution under the current DBL of three fish. We then used the fitted negative binomial to predict the daily harvest distribution under a DBL of five fish. Using this approach, the potential increase in the number of fish harvested with the higher DBL was minimal, but the additional harvest showed an increasing 1997–2005 trend from 315 to 7,301 fish for charter anglers and from 450 to 18,151 fish for noncharter anglers. There was a positive linear relationship between proportion of anglers harvesting the current DBL and harvest rates, which indicates that angler success is a function of Chinook salmon abundance. Using an age‐structured deterministic population model and the production‐conversion efficiency method, we estimated potential reduction in annual lakewide consumption of alewives by Chinook salmon resulting from a higher DBL. The reduction in consumption averaged 313 metric tons and increased from 28 (1997) to 935 metric tons (2005). We conclude that higher DBLs would have a relatively minor impact on lakewide forage fish abundance but would be more consistent with current management objectives.
