No abstract

When four species of fish were taken from western Lake Erie in each of four seasons and held usually for less than 7 days at ambient lake temperatures, the temperatures they selected during 2–3 days in a horizontal temperature gradient differed seasonally. The differences were largely attributable to the conditions at which the fish had been acclimatized in the lake, and were modified by acclimation during 2–3 days in the gradient.The selected temperatures provided insights into the temperatures that might be selected by these species each season if the lake basin or other waters with similar seasonal ambient temperatures were subjected to thermal discharges. Temperatures selected were above ambient lake temperatures except for emerald shiners (Notropis atherinoides) in summer and fall. In general, white bass (Morone chrysops) and smallmouth bass (Micropterus dolomieui) selected a high range in temperatures throughout the year (18–30 C and 18–31 C, respectively), yellow perch (Perca flavescens) an intermediate range (10–29 C) and emerald shiners the lowest range (6–23 C). Three of the species were distributed within a relatively precise temperature range in the summer and within a larger range during other seasons; emerald shiners selected a narrow range during all seasons. A fairly stable temperature preference was usually reached within several hours in summer, but the temperatures selected by three species generally increased with time in the gradient during the other seasons; emerald shiners selected constant temperatures in all seasons. Temperatures selected by young and adults differed mainly in yellow perch and emerald shiners in summer and winter, when the lake temperatures fluctuated least.

We examined conditions under which white shrimp (Litopenaeus setiferus) post-larvae enter an estuarine channel receiving high freshwater discharge and one receiving negligible discharge in the Ossabaw Sound system of Georgia, USA, during 1997 and 1998. We used surface nets to collect plankton over several 14-day periods, during which consecutive tows were made at night against the flooding current at stations in the inlet channels. During these sampling periods, additional intensive periods of around-the-clock surface and near-bottom (using a bottom sled) plankton tows were made. Data on oceanographic conditions were obtained from moored instrument arrays and shipboard sampling. We identified three key factors that influenced the densities of post-larval white shrimp in time and space within the Ossabaw inlet system. The first factor was a critical minimum temperature of coastal waters of 27-28°C. Once the threshold temperature was reached, lunar tidal stage became a key factor when the full duration of the flood tide coincided with darkness during peak ingress. This peak also coincided with an increase in water level within the system by more than 0.2 m, which induced an additional influx of water that reinforces the flood current over the ingress period. Our results suggest that the direction of subtidal currents (into or out of the system) becomes a significant factor in post-larval ingress when influx of water coincides with the time of favorable temperature conditions and nighttime flood tides.

No abstract

Before the gears were used for catch comparisons, a two-seam net and a tongue trawl were evaluated for changes in net dimensions with fishing depth and tow direction. When towed as it would be during catch comparisons, the two-seam net had a width of 16.1 m and was estimated to extend 2.1 m vertically at the center of the head rope. The hori/ontal spread of the tongue trawl was 13.5 m and its vertical spread was 4.2 m at center. Small, statistically consistent differences in openings (<0.5 m) occurred with depth and direction. The major factor influencing changes in catch (kg/ha) with depth (10-fold increase in shallow water) appeared to be the fauna! distribution with depth, independent of towing characteristics. Differences in biomass (kg/tow, kg/ha), and in the biomass (kg/ha) ratios of taxa to shrimp between the two-seam and tongue trawls were documented for eight major biological groups. Major differences in total catch by net occurred between years primarily because of changes in the catch of miscellaneous invertebrates and shrimp. Significant differences in the lengths of nine priority species occurred between the two gears. Mean lengths in the two nets differed by more than 1 cm for spot Leiostotnus xanthurus (which was larger in the tongue net), Atlantic croaker Micropogonias undulatus (larger in the two-seam net), and Spanish mackerel Scomberomorus maculatus (larger in the tongue net). Mean ratio of fish to shrimp biomass was 31:1 overall (21:1 for the two-seam net and 41:1 for the tongue trawl). Ratios of total biomass and the biomass of any taxonomic grouping to shrimp biomass did not differ statistically between the two gears. Biomass ratios were recalculated from published data by a standard methodology. Subsequent comparisons indicated increases in the ratios over time and highlighted a need to validate the technique of subsampling heterogenous trawl samples. Finfish by-catch in both gears was dominated by sciacnids (44% by weight of all fish). Red drum Sciaenops ocellatus, spotted scatrout Cynoscion nebulas us, snappers (Luljanidac), and groupers (Epinephelinae) were not caught by cither net. Catches of Spanish mackerel and king mackerel Scomberomorus cavalla were documented and warrant further investigation to evaluate the effects of by-catch on local populations.