Comparison of Push Nets and Tow Nets for Sampling Larval Fish with Implications for Assessing Littoral Habitat Utilization

—Understanding sampling effectiveness is critical to gear selection and the determination of larval fish dynamics. We evaluated bow-mounted push nets for collecting larval fish across reservoirs and habitats and compared them with traditional tow nets. By means of a pushed 0.5-m-diameter conical net and towed 0.5-m-diameter and 0.75-m-diameter conical nets, ichthyoplankton samples were taken during daylight hours from May to July on 21 reservoirs that varied in morphological and environmental characteristics. The push net had higher catches than the same-diameter tow net. However, the push net was not as efficient as the larger-sized tow net in July, when larvae are larger. For pelagic habitats, bow-mounted push nets or large tow nets will sample the larval fish community more efficiently than traditional 0.5-m-diameter conical tow nets. We also assessed push nets for sampling nearshore littoral habitats (,1.0 m in depth). Across reservoirs the littoral areas had much higher catch rates than did the offshore pelagic zone; estimates of peak larval fish densities were four times as high in the littoral zone. Deriving estimates of larval fish abundance from pelagic habitats only will probably lead to underestimation of total larval fish densities. The versatility of the push net in sampling littoral habitats is an important consideration when designing surveys to estimate larval fish communities. Bow-mounted push nets can be used to effectively sample both pelagic and littoral larval fish communities, whereas traditional tow nets are only suited for pelagic habitats. Estimates of larval fish abundance and size structure are important in understanding year-class strength, and population models should include a component of prerecruitment surveys (Walters and Collie 1988). Accurate estimates of densities and size distributions of larval fish are critical in assessing early life history dynamics and several types of ichthyoplankton gears (seines, dip nets, light traps, push nets, and tow nets) are employed in fisheries assessments with varying efficiencies (Choat et al. 1993; Hale et al. 1995; Isermann et al. 2002). Understanding the strengths and limitations associated with the different gear types is critical to interpreting estimates of larval fish densities and size distributions. The size of the net mouth (Barkley 1972; Hale et al. 1995), size of the net mesh (Colton et al. 1980; Jessop 1985; Isermann et al. 2002), and sampling speed (Barkley 1972; Colton et al. 1980) have been shown to affect catch of different species and sizes of larval fish. Other environmental variables such as water clarity, turbulence (caused by boat wash), * Corresponding author: [email protected] 1 Present address: Michigan Department of Natural Resources, Charlevoix Fisheries Research Station, 96 Grant Street, Charlevoix, Michigan 49720, USA. Received August 1, 2003; accepted June 25, 2004 Published online February 28, 2005 and net-clogging due to plankton can also affect net efficiency (Anderson et al. 1998). Traditional towed nets are commonly used to sample larval fish, but bow-mounted push nets of the same size may reduce gear avoidance bias and influences of boat propeller wash for ichthyoplankton surveys. Previous studies comparing push and towed nets have used different sized nets (Gallagher and Conner 1983; Hale et al. 1995), so direct comparison of the sampling bias of these two designs was not possible. Information on the efficiency and utility of push nets compared with traditional towed nets will improve our ability to accurately estimate larval fish densities and size distributions. Studies aimed at investigating the early life history of fishes should include sampling of a wide range of species, larval sizes, and habitats. However, sampling is often focused in the pelagic zone where traditional gears such as tow nets can be easily used to assess ichthyoplankton and avoid the difficulties of sampling shallow (,1.0 m) habitats (e.g., Sammons and Bettoli 1998). Several gear types developed to sample shallow water, such as the larval seine (Leslie et al. 1983), are effective in littoral habitats, but they tend to be qualitative and cannot be compared directly with pelagic gears. Because fishes found in reservoirs and inland lakes typically spawn inshore (Auer 1982), the potential for misrepresenting population 87 COMPARISON OF PUSH NETS AND TOW NETS levels is high without information on ichthyoplankton abundance and diversity in nearshore areas. Therefore, the objective of this study was to describe differences in catches across habitats between traditional towed ichthyoplankton nets and push nets.