We developed a Kemp's ridley (Lepidochelys kempii) stock assessment model to evaluate the relative contributions of conservation efforts and other factors toward this critically endangered species' recovery. The Kemp's ridley demographic model developed by the Turtle Expert Working Group (TEWG) in 1998 and 2000 and updated for the binational recovery plan in 2011 was modified for use as our base model. The TEWG model uses indices of the annual reproductive population (number of nests) and hatchling recruitment to predict future annual numbers of nests on the basis of a series of assumptions regarding age and maturity, remigration interval, sex ratios, nests per female, juvenile mortality, and a putative ''turtle excluder device effect'' multiplier starting in 1990. This multiplier was necessary to fit the number of nests observed in 1990 and later. We added the effects of shrimping effort directly, modified by habitat weightings, as a proxy for all sources of anthropogenic mortality. Additional data included in our model were incremental growth of Kemp's ridleys marked and recaptured in the Gulf of Mexico, and the length frequency of stranded Kemp's ridleys. We also added a 2010 mortality factor that was necessary to fit the number of nests for 2010 and later (2011 and 2012). Last, we used an empirical basis for estimating natural mortality, on the basis of a Lorenzen mortality curve and growth estimates. Although our model generated reasonable estimates of annual total turtle deaths attributable to shrimp trawling, as well as additional deaths due to undetermined anthropogenic causes in 2010, we were unable to provide a clear explanation for the observed increase in the number of stranded Kemp's ridleys in recent years, and subsequent disruption of the species' exponential growth since the 2009 nesting season. Our consensus is that expanded data collection at the nesting beaches is needed and of high priority, and that 2015 be targeted for the next stock assessment to evaluate the 2010 event using more recent nesting and in-water data.
Since the introduction of striped bass Morone saxatilis and hybrids of striped bass and white bass Morone chrysops into reservoirs, much concern has been directed at the possibility of these predators competing with other sport fishes for limited prey. If density of striped bass is reduced or eliminated through modifications of the stocking program, the prey not consumed by striped bass may be shifted to other sport fishes. The resulting increase in biomass of other sport fishes would be a function of the amount of added prey, the percent of this additional prey eaten by other sport fishes, and the efficiency with which the prey is converted into biomass. We used bioenergetics models to estimate annual striped bass prey consumption in Norris Reservoir, Tennessee. Total annual consumption was estimated at 52 kg/ha (estimated range ϭ 17-100 kg/ha), clupeids accounting for the majority (94%), followed by lepomids (4%) and other food items (2%). Existing biomass of black basses Micropterus spp., crappies Pomoxis spp., and percids Stizostedion spp. was about 65 kg/ha (estimated range ϭ 35 Ϫ 106 kg/ha). Given the complete removal of striped bass, modeling indicated that the most probable increase in the biomass of these sport fishes would be about 3% with a 75% probability that it would be less than 12%. Thus, not even the complete removal of striped bass would measurably increase the biomass of other sport fishes.
Various designs of low-head dams are used to rehabilitate streams or forestall upstream channel incision after channelization. We report on the efficacy of using notched sills and grade control structures (GCS) to restore the fish assemblage in Luxapallila Creek, Mississippi. We tested the null hypotheses that habitat variables and species richness, evenness, and assemblage structure would not differ among: (1) a channelized segment with no modifications; (2) a channelized segment mitigated by the installation of sills and GCS; (3) a segment upstream of the installations and undergoing channel incision; and (4) an unaltered segment. Although habitat variables changed, neither species richness, evenness, nor fish assemblage structure differed between mitigated and channelized segments with both exhibiting less richness and different assemblage structures than the unaltered segment. Lack of differences in species richness between the incised and unaltered segments suggest that the GCS may have halted the negative effects of upstream channel incision before species were extirpated. Conspicuous habitat differences between the altered (channelized and mitigated) and unaltered segments were lack of backwaters and canopy coverage and finer substrates in the altered segments. Our results suggest a more comprehensive rehabilitation strategy is required in
Diet overlap and consumption patterns suggest seasonal flux in the likelihood for exploitative competition among piscivorous fishes Un resumen en españ ol se incluye detrás del texto principal de este artículo.
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