Age-0 largemouth bass Micropterus salmoides were studied in Bay Springs Reservoir, Mississippi, to determine if the larger young had greater survival through their first winter and if this survival was influenced by levels of energy reserves. Abundance decreased from 542 young/ hectare in June, when fish averaged 37 mm total length, to 12/hectare in March, when lengths averaged 149 mm. The length-frequency distribution of this year-class was positively skewed during June-August, became bimodal during September-January', and lost most of the smaller mode by March. During September-January, fish in the smaller mode decreased in abundance from 176 to 9/hectare and their mean length remained near 60 mm; fish in the larger mode decreased in abundance from 76 to 27/hectare and increased in length from 108 to 154 mm. Whole-body lipid content increased from 3.5% of dry body weight in fall to 5.9% in December and decreased to 3.3% in March. All young had equal amounts of stored lipid by late fall, but by March the surviving small young had less lipid than large young, suggesting that during winter smaller fish spent their reserves at a faster rate. Protein content averaged 74.6% of the dry body weight and neither differed among months nor changed with fish length. Mortality of the smallest young was accelerated in late fall and winter, coinciding with decreases in lipid stores and rapid drops in water temperature.

Fish size and electrical waveforms have frequently been associated with variation in electrofishing effectiveness. Under controlled laboratory conditions, we measured the electrical power required by five electrical waveforms to immobilize eight fish species of diverse sizes and shapes. Fish size was indexed by total body length, surface area, volume, and weight; shape was indexed by the ratio of body length to body depth. Our objectives were to identify immobilization thresholds, elucidate the descriptors of fish size that were best associated with those immobilization thresholds, and determine whether the vulnerability of a species relative to other species remained constant across electrical treatments. The results confirmed that fish size is a key variable controlling the immobilization threshold and further suggested that the size descriptor best related to immobilization is fish volume. The peak power needed to immobilize fish decreased rapidly with increasing fish volume in small fish but decreased slowly for fish larger than 75–100 cm3. Furthermore, when we controlled for size and shape, different waveforms did not favor particular species, possibly because of the overwhelming effect of body size. Many of the immobilization inconsistencies previously attributed to species might simply represent the effect of disparities in body size.

The alluvial valley of the lower Mississippi River contains hundreds of fluvial lakes that are periodically connected to the river during high water, although the frequency, duration, and timing of the connections vary. To help design plans to restore and preserve fish assemblages in these alluvial lakes, this investigation tested whether predictable patterns in lake fish assemblages were linked to the level of connectivity with the river. Results suggested that connectivity played an important role in structuring fish assemblages and that it was correlated with variables such as lake size, depth, distance from the river, and age, which exhibit a continuum of predictable features as the river migrates away from abandoned channels. Annual floods homogenize the floodplain and promote connectivity to various degrees, allowing for fish exchanges between river and floodplain that directly affect fish assemblages. The major physical changes linked to reduced connectivity are loss of depth and area, which in turn affect a multiplicity of abiotic and biotic features that indirectly affect community structure. In advanced stages of disconnection, fish assemblages in oxbow lakes are expected to include largely species that thrive in turbid, shallow systems with few predators and low oxygen content. When the river flowed without artificial restraint, oxbow lakes were created at the rate of 13–15 per century. At present, no or few oxbow lakes are being formed, and as existing lakes age, they are becoming shallower, smaller, and progressively more disconnected from the river. Given that modifications to the Mississippi River appear to be irreversible, conservation of this resource requires maintenance of existing lakes at a wide range of aging phases that provide diverse habitats and harbor distinct species assemblages.

We used an equilibrium yield model to simulate the effect of reducing exploitation on yield and average weight of white crappies Pomoxis annularis, based on empirical growth data and various levels of conditional natural mortality. Modeling indicated that reducing exploitation would likely increase yield as fish growth increased and natural mortality decreased; however, reducing exploitation would not substantially affect yield when conditional natural mortality exceeded 30–40%, regardless of growth. For instance, a 250‐mm minimum length limit provided higher yield than a 200‐mm minimum length limit only if growth was above average and conditional natural mortality was less than 30–40%. Average weight of crappies harvested increased with growth and length at recruitment to the fishery but decreased with exploitation and conditional natural mortality. Our results suggest that a length limit could improve both yield and average weight for crappie fisheries only if growth is rapid and natural mortality is low. If natural mortality is not low, a length limit could improve average weight of fish harvested without substantially reducing yield.

Winter mortality of age‐0 largemouth bass Micropterus salmoides is sometimes size dependent, with smaller fish experiencing higher mortality. We conducted this study to determine if the presence of predators influenced winter mortality of young largemouth bass, if predators influenced all sizes of young equally, and if increased shelter availability moderated a possible relation between predator‐induced mortality and fish size, We stocked 0.06‐ha experimental ponds with largemouth bass (30 fish/pond) of five length groups (55–100, 101–125, 126–150, 151–175, and 176–200 mm total length), with and without predators (three largemouth bass 250–350 mm long), and four levels of shelter (0, 10, 16, and 26% brush coverage of surface area of ponds). In ponds without shelter, survival ranged from 10 to 97% in the presence of predators and from 77 to 93% in the absence of predators. Fish less than 126 mm long had gradually lower survival in the presence of predators, but near 80% survival in the absence of predators; fish 126 mm long or longer had more than 80% survival in the presence or absence of predators. In ponds with predators and shelter, survival increased with fish length and amount of shelter, but the lifesaving value of increased length and shelter relative to winter survival faded in fish 126 mm or more in length. We suggest that in lower latitudes predators may be a major source of mortality of small age‐0 largemouth bass in winter, and that the effects of predators can be tempered by shelter.

The compensatory mortality hypothesis postulates that a population's total mortality remains unchanged at low to intermediate exploitation rates because natural mortality decreases to compensate for reduced density, whereas the additive mortality hypothesis postulates that any increase in exploitation mortality results in an increase in total mortality. Fishing and natural mortality rates have generally been assumed to be additive rather than compensatory. We reviewed mortality estimates for some prominent sportfish populations to identify evidence for compensatory or additive mortality. For largemouth bass Micropterus salmoides, total annual mortality increased with annual exploitation suggesting additive mortality. For crap pies Pomoxis nigromaculatus and I? annularis, annual exploitation did not seem to affect annual mortality at low to moderate annual exploitation, but annual mortality increased with annual exploitation as it increased beyond 40%, conforming to the compensatory mortality hypothesis. Northern pike Esox lucius mortality estimates revealed no relation between annual mortality and annual exploitation, and highly variable annual mortality for a given annual exploitation. Evidence from the literature suggests that mortality of northern pike may be compensatory for fish smaller than 40 cm total length, but additive for larger fish. Because compensatory natural mortality reduces the managers' ability to control annual mortality, we suggest that further consideration of the compensatory mortality hypothesis be given to species that have shown variable mortality responses to reductions in annual exploitation.

Over 6,000 crappies Pomoxis spp. were tagged in five water bodies to estimate exploitation rates by anglers. Exploitation rates were computed as the percentage of tags returned after adjustment for three sources of uncertainty: postrelease mortality due to the tagging process, tag loss, and the reporting rate of tagged fish. Confidence intervals around exploitation rates were estimated by resampling from the probability distributions of tagging mortality, tag loss, and reporting rate. Estimates of exploitation rates ranged from 17% to 54% among the five study systems. Uncertainty around estimates of tagging mortality, tag loss, and reporting resulted in 90% confidence intervals around the median exploitation rate as narrow as 15 percentage points and as broad as 46 percentage points. The greatest source of estimation error was uncertainty about tag reporting. Because the large investments required by tagging and reward operations produce imprecise estimates of the exploitation rate, it may be worth considering other approaches to estimating it or simply circumventing the exploitation question altogether.

No abstract