Christopher M. Legault

Population Viability Analysis of Atlantic Salmon in Maine, USA

Abstract Populations of Atlantic salmon Salmo salar in eight rivers of Maine, USA, are listed as endangered under the U.S. Endangered Species Act. This listing has required the creation of measurable and objective delisting criteria. One component of these criteria is the determination of the abundance levels associated with recovered populations. Population viability analysis (PVA) was chosen as the method to estimate these recovery levels because it (1) formalizes the combination of information available on the species. (2) quantifies the uncertainty in the population parameters and evaluates the impact of this uncertainty on the probability of extinction, and (3) allows examination of potential management strategies. A PVA specific to the Atlantic salmon populations in Maine was conducted based on the life history characteristics of the endangered populations (i.e., multiple ages of return from sea, kelting, river-specific habitat limitations, and use of stocking as a recovery tool). The model was veri...

Marine growth and morphometrics for three populations of Atlantic salmon from eastern Maine, USA

Abstract Significant differences in growth and prespawning body morphology were detected among three stocks of Atlantic salmon reared in a common marine environment. Smolts originating from river-specific broodstock of the Machias, East Machias, and Dennys populations were reared at two marine net-pen facilities for 25 months. Significant differences in stock-specific growth were observed among two stocks at both sites, suggesting a genetic basis for the observed phenotypic variation. There was a significant stock effect to the total measured phenotypic variation based on collected truss network analyses. Linear discriminant function analysis of a truss network of morphometric distances allowed for 73% accuracy of stock classification. A thin-plate spline procedure characterized the Machias body form as having a shortened narrow caudal peduncle region, a compressed body with an elongated trunk, and a deeper head region relative to the other two stocks. Phenotypic variation may be associated with hydrologi...

Evaluation of Removal Sampling for Basinwide Assessment of Atlantic Salmon

Abstract Removal estimators for stream fish abundance are widely used but can result in biased population estimates at the site level. We conducted computer simulations to examine how the Carle and Strub (1978) estimator, coupled with variation in catchability, influences the accuracy of population estimates at the site level. Site-level population estimates were then used to examine what effect potential bias in the population estimate at a site had on basinwide abundance estimates. Historic electrofishing data collected from Atlantic salmon Salmo salar in the Narraguagus River, Maine, were used as the baseline for construction of these simulations. At the site level, mean percent bias of population estimates was −23% when catchability was low (0.30–0.40) and when the true population was low (1–20 fish). Bias was reduced as the true population size increased and catchability increased. The negative bias at the site level affected total population estimates for the entire river basin. Under current sampli...

Mixed and Multi-Stock FisheriesIntroduction

The September 2003 ICES Annual Science Conference was held in Talinn, Estonia, and Theme Session V was ‘‘Mixed and multi-stock fisheries e challenges and tools for assessments, prediction, and management.’’ The theme session brought together researchers from the salmonid and marine fishery fields to address common problems faced when multiple fleets harvest a common stock, or multiple stocks are harvested by a common fleet. In all, 23 papers were presented during the theme session, prefaced by an invited plenary lecture by Randall Peterman that focused on challenges facing fisheries scientists and managers. The ten papers that follow represent the diversity of topics in the Theme Session, but all focus on approaches to overcome the uncertainty inherent in these complex situations. Fleets and fisheries that harvest multiple stocks, and stocks harvested by multiple fisheries, face a number of complexities. Less productive stocks in a mix of stocks may suffer unsustainable mortality, while more-productive stocks continue to support sustainable catches. Stocks harvested together may be at a very different status relative to safe biological limits, which would make different harvest strategies necessary in the same fisheries. Where fisheries take multiple stocks of the same species, uncertainties about allocating catches to stocks present special assessment problems. Projections to guide advice on effort levels appropriate for different stocks in the harvested complex can also thwart traditional approaches, and can produce results that are either incompatible across stocks, or not straightforward for managers to apply. Therefore, the objectives for Theme Session V were to describe developments in assessment and projection methods for multi-stock and multi-fleet fisheries, and to discuss opportunities for applying new concepts more widely. Management of complex systems is not necessarily facilitated by making stock assessment and management more complex. Ideally, simple rules should be devised that are robust to the pervasive uncertainties in such systems. However, the process by which these simple rules could be found would incorporate as much complexity as possible.

Relative importance of population size, fishing pressure and temperature on the spatial distribution of nine Northwest Atlantic groundfish stocks

The spatial distribution of nine Northwest Atlantic groundfish stocks was documented using spatial indicators based on Northeast Fisheries Science Center spring and fall bottom trawl survey data, 1963–2016. We then evaluated the relative importance of population size, fishing pressure and bottom temperature on spatial distribution with an information theoretic approach. Northward movement in the spring was generally consistent with prior analyses, whereas changes in depth distribution and area occupancy were not. Only two stocks exhibited the same changes in spatiotemporal distribution in the fall as compared with the spring. Fishing pressure was the most important predictor of the center of gravity (i.e., bivariate mean location of the population) for the majority of stocks in the spring, whereas in the fall this was restricted to the east-west component. Fishing pressure was also the most important predictor of the dispersion around the center of gravity in both spring and fall. In contrast, biomass was the most important predictor of area occupancy for the majority of stocks in both seasons. The relative importance of bottom temperature was ranked highest in the fewest number of cases. This study shows that fishing pressure, in addition to the previously established role of climate, influences the spatial distribution of groundfish in the Northwest Atlantic. More broadly, this study is one of a small but growing body of literature to demonstrate that fishing pressure has an effect on the spatial distribution of marine resources. Future work must consider both fishing pressure and climate when examining mechanisms underlying fish distribution shifts.