Transactions of the American Fisheries Society

The Alverson and Carney (AC) model for estimating the instantaneous rate of natural mortality (M) was reformulated to include β andt₀parameters. The revised Zhang and Megrey (ZM) model could be used for the estimation ofMinstead of the AC model since von Bertalanffy and allometric growth parameters are readily available for most exploited fish stocks. It was determined from the new formulation that the ratios of the age at maximum biomass (t_mb) to the maximum age (t_max) for pelagic and demersal species were significantly different from 0.38, the value originally proposed by Alverson and Carney. The ratios for these two ecological groups were 0.302 and 0.440, respectively, and were significantly different. We examined the sensitivity and bias inMfrom the new formulation relative to those from the AC model, which assumed that β = 3.0 andt₀= 0. Estimates ofMfrom the AC model are most sensitive to the assumption that growth starts att₀= 0 when growth rates are high as well as to the β andt_mbassumptions. The performance of the revised ZM model was evaluated by comparing calculatedMvalues from the two models based on a paired‐samplet‐test. The results of the two statistical analyses showed that the ZM model produced values ofMcloser to published estimates than did the AC model. Thus, the ratiot_mb/t_maxfor specific ecological groups should be used rather than Alverson and Carney's constant 0.38. Analyses of exploited stock dynamics might be conducted using the possible range ofMinstead of the constant value. The range could be estimated from the ZM model using the mean ratiot_mb/t_max± SD to get thet_mbfor each subgroup or by explicit variance calculations.

On Campeche Bank (CB) in the southern Gulf of Mexico, Gags Mycteroperca microlepis are captured as part of a sequential multispecies fishery. Limited scientific data suggest that the Gag is the third most important commercial species in the region, contributing 21% (total weight) of the overall grouper catch. Neither age nor stock data are currently available for Gags in this region, making it impossible to determine how this species is affected by current fishing practices. Such information is vital for the CB stock since it acts as a genetic source for a mixed Gag population on the west Florida shelf through a constant unidirectional flow of larvae. The goal of this study was to generate baseline age and stock structure data for Gags on CB. Age was quantified from otoliths by counting the number of annuli (indicated by the number of opaque zones from the otolith core to the margin) in thin sections. Otolith shape was analyzed by using discrete wavelet transforms, linear otolith morphometrics, and shape indices. In each of 198 specimens, age was determined based on examination of the right sagittal otolith, and shape analysis was conducted on the left sagittal otolith. Individual age ranged from 4 to 17 years, and otolith opaque band deposition occurred annually between December and February. Using principal components analysis and linear discriminant analysis, two distinct groups were identified (discrimination success = 98%), indicating the presence of substructure in the CB stock of Gags. Otolith analysis clearly assisted in achieving a better understanding of the CB Gag stock and in identifying the complexity within the stock, demonstrating that otolith analysis is a useful tool in characterizing fish populations. The study results highlight the urgent need for Gag stock assessment in this region and for the application of a cautious approach to fishery management given the immense gaps in biological data for this species.

Received January 13, 2016; accepted July 18, 2016 Published online October 7, 2016

Mislabeling of North American merlucciid hakes in stock surveys and commercial market samples was detected by employing nuclear 5S ribosomal DNA (rDNA) and mitochondrial cytochrome b variation as molecular markers. Results showed that offshore hake Merluccius albidus is sold in European markets but is labeled as the morphologically similar silver hake M. bilinearis, which is the target species of the fishery. This suggests that offshore hake may be inadvertently included within silver hake landings, as the two species overlap in the southern area of silver hake distribution (approximately 41°‐35°N latitude near North American coasts). An inexpensive and technically easy technique based on polymerase chain reaction (PCR) amplification of a fragment of 5S rDNA and visualization of PCR products in agarose gels is recommended for routine species assignation in landings for purposes of exploitation estimates and for authentication of commercial hake species.

Dams created by North American beavers Castor canadensis (hereafter, “beavers”) have numerous effects on stream habitat use by trout. Many of these changes to the stream are seen as positive, and many stream restoration projects seek either to reintroduce beavers or to mimic the habitat that they create. The extent to which beaver dams act as movement barriers to salmonids and whether successful dam passage differs among species are topics of frequent speculation and warrant further research. We investigated beaver dam passage by three trout species in two northern Utah streams. We captured 1,375 trout above and below 21 beaver dams and fitted them with PIT tags to establish whether fish passed the dams and to identify downstream and upstream passage; 187 individual trout were observed to make 481 passes of the 21 beaver dams. Native Bonneville Cutthroat Trout Oncorhynchus clarkii utah passed dams more frequently than nonnative Brown Trout Salmo trutta and nonnative Brook Trout Salvelinus fontinalis. We determined that spawn timing affected seasonal changes in dam passage for each species. Physical characteristics of dams, such as height and upstream location, affected the passage of each species. Movement behaviors of each trout species were also evaluated to help explain the observed patterns of dam passage. Our results suggest that beaver dams are not acting as movement barriers for Bonneville Cutthroat Trout or Brook Trout but may be impeding the movements of invasive Brown Trout.

The activity of beavers Castor canadensis in freshwater environments can have considerable localized impacts on the physical and biological components of riparian ecosystems. By changing the habitat of a stream, beaver dams can cause spatial variation in growth opportunity that may have direct consequences for the growth of resident fish. In a small stream in eastern Canada, we studied the effects of an ephemeral beaver pond on the growth and maturity of age‐2 Atlantic salmon Salmo salar parr tagged with passive integrated transponder tags. Water temperature remained relatively uniform throughout the study site. We found very little movement of recaptured fish in the study site. Fish that were recaptured in the beaver pond displayed faster summer growth rates in both length and mass than fish that were recaptured immediately above or below the pond. We also found that parr in the pond maintained relatively high condition factors, whereas fish above and below the pond appeared to decrease in condition factor throughout the summer. In addition to growth, the maturation rates of age‐2 males were higher above the dam than below. This study demonstrates the effect a beaver dam can have on individual growth rates. By influencing growth during sensitive periods, the beaver pond may also influence individual life history pathways. This information could be an important component in ecosystem models that predict the effect of beaver population dynamics on the growth of individual salmonids at the landscape scale.

Climate and hydrologic variability are defining characteristics of California rivers. Recently, the region experienced an unprecedented drought, and the probability of similarly warm, dry conditions is predicted to increase. In addition to warming air and water temperatures, climate change projections predict increased flooding and sea level rise, likely aggravating the water resource issues that already challenge the western United States. Water managers balance many public interests, including the conservation of native fishes, such as the Chinook Salmon Oncorhynchus tshawytscha. Given projected changes in climate and hydrology, there is an urgent need to understand how salmon respond to these conditions. In this study, we examined how young salmon responded to extreme drought (2012–2015) versus flood (1998–1999) conditions in the Yolo Bypass, a floodplain–tidal slough complex of the Sacramento River, California. We found that the diets of juvenile Chinook Salmon were dominated by aquatic–riparian insects during flooding and were dominated by zooplankton during the drought. Although juvenile salmon that were caught during the drought seemed to have eaten a higher number of prey items on average, they also had higher metabolic costs. Therefore, it is likely that juvenile salmon must augment their foraging behavior to offset higher temperatures and prey shifts. Finally, preferentially consumed, calorically valuable prey (i.e., larger zooplankton and aquatic–riparian insects) have become rare due to habitat degradation and biological invasions, and resource managers must consider re‐establishing productive off‐channel habitats, such as riparian corridors, floodplains, and wetlands.

In the late 1990s the Environmental Monitoring and Assessment Program of the U.S. Environmental Protection Agency developed a structured set of tests to evaluate and facilitate selection of metrics for indices of biotic integrity (IBIs). These IBIs were designed to be applicable across multistate regions as part of a national assessment of all U.S. waters. Here, we present additional steps in, and refinements to, that IBI development process. We used fish and amphibian assemblage data from 932 stream and river sites in 12 western U.S. states to develop IBIs for Mountains, Xeric, and Plains ecoregions. We divided 237 candidate metrics into nine metric classes representing different attributes of assemblage structure and function. For each ecoregion we sequentially eliminated metrics by testing metric range, signal‐to‐noise ratios, responsiveness to disturbance, and redundancy to select the best metric in each class. The IBIs for the Mountains and Plains each had seven metrics and the Xeric IBI had five. In the Mountains, half of the estimated stream length that could be assessed had IBI scores greater than 62 (out of 100). In the Xeric and Plains, half the stream length had scores no greater than 50 and no greater than 37, respectively. An estimated 16% of Xeric stream length had scores greater than 62 (the median for the Mountains), while 5% of Plains stream length had scores that exceeded 62. This IBI development process is less subjective and more streamlined and has more clearly defined criteria for metric selection and scoring than those used in the past, while maintaining a strong ecological foundation.

Energy exploration in the Gulf of Mexico (hereafter, Gulf) has resulted in the addition of numerous oil and gas production platforms that create structurally complex habitat in an area otherwise dominated by barren mud/sand bottom. How these artificial structures affect fish populations is largely unknown, and there is ongoing debate regarding their value as surrogate habitats for ecologically and economically important reef fish species. Thus, the purpose of this study was to characterize trends in Red Snapper Lutjanus campechanus reproductive potential in the western Gulf at oil and gas platform reefs relative to reproductive potential at natural banks. Red Snapper (n = 1,585) were collected during 2013–2015 from standing platforms, decommissioned platform artificial reefs, and natural banks by using standardized vertical line gear. Comparisons of gonadosomatic index, male : female ratios, batch fecundity, annual fecundity, spawning frequency, and number of spawning‐capable individuals indicated that Red Snapper reproductive biology was similar among natural bank, standing platform, and artificial reef habitats. These results suggest that in terms of reproductive output, fish inhabiting artificial reefs are functionally similar to similar‐aged fish on natural banks. This work can be used to make informed management decisions and suggests that there are benefits to converting decommissioned platforms into designated artificial reefs. Future studies should consider site‐specific characteristics, such as depth, vertical relief, and proximity to other structures, to elucidate how habitat characteristics may influence reproduction, ultimately improving future artificial reef deployments for fisheries enhancement in the western Gulf.

Diets of red snapper Lutjanus campechanus were compared among four diel feeding periods (dawn, day, dusk, and night) and among three standard length size‐classes: small (200–299 mm), medium (300–399 mm), and large (400–499 mm). Fish were collected from August through October 2000 in the north‐central Gulf of Mexico. A total of 432 stomachs were examined, of which 164 (37%) contained prey. Size‐class influenced diets; diets of small red snapper were dominated by fish, those of medium red snapper by fish and tunicates, and those of large red snapper by crabs and fish. Red snapper fed on prey from several different habitats. Small red snapper fed mostly on reef and sand prey types; medium red snapper fed on similar portions of reef, sand, and mixed prey types; and large red snapper fed on mixed prey types. Adjusting for fish size, mean gut fullness was significantly lower for the dusk than for the day period. Fish were the most important prey for all periods. The second most important prey group changed with period: stomatopods for dawn, tunicates for day, and crabs for dusk and night periods. Prey habitat type also varied over periods. Sand prey type dominated dawn and dusk periods, whereas reef prey type dominated day and night periods. We used multidimensional scaling analysis to compare the relative influence of diel period to size‐class on red snapper diets. This analysis suggested that fish size‐class had a greater influence on red snapper diet than did period. However, with size effects removed, red snapper still showed significant diet shifts with periods.