Efficacy of minnow traps for sampling mummichogs in tidal marshes

We evaluated two types of wire-mesh minnow traps for sampling mummichogs (Fundulus heteroclitus) from intertidal rivulets on the surface of a salt marsh. In field trials, mean catch (no. of fish trap⁻¹) did not differ significantly between trap types nor across soak times ranging from 30 to 240 min. Catch rate (no. of fish trap⁻¹ min⁻¹) was significantly (K-W ANOVA on ranked data, H_4df=24.79; p<0.001) greater at shorter soak times, and larger size classes (>45 mm TL) of fish were relatively more abundant (Kolmogorov-Smirnov test, p<0.0001) in collections from double-funnel cylindrical traps (volume=10,330 cm³) compared to rectangular traps having about twice the volume and a single funnel cylindrical traps (volume=10,330 cm³) compared to rectangular traps having about twice the volume and a single funnel opening. We estimated entry and escape rates by adding marked fish to traps at initial densities of 30, 60, and 120 trap⁻¹ and measuring changes in the numbers of marked and unmarked fish in each trap type at soak times ranging from 15 to 120 min during the flood stage of the tidal cycle. Escape rate was almost, twice as great from the two-funnel cylindrical traps (0.77 fish min⁻¹) than from the single-funnel rectangular type (0.42 fish min⁻¹) but entry rates did not differ significantly (K-W ANOVA, H_4df=0.06; p=0.813) between trap types. Escape rate increased and entry rate decreased as fish density within traps was increased. Escape rates were highest immediately following immersion of the traps, but approached a constant value after 30 min. Trap-specific rates of entry and escape can be affected by a variety of factors including physical characteristics of different trap types, method and timing of trap deployment, and fish behavior. Catch data from minnow traps are unlikely to be useful in measuring relative abundance of mummichogs among habitats unless very short soak times (≤60 min) are used and calibration tests are performed. Use of longer soak times virtually ignores the dynamics involved in determining catch, and is unlikely to provide accurate information on either abundance or size distribution of mummichogs from tidal marsh habitats.