Anadromous Sturgeons: Habitats, Threats, and Management

<em>Abstract</em>.—Space use and habitat selection of early juvenile Atlantic sturgeon <em>Acipenser oxyrinchus</em> have been little studied and remain largely unknown throughout the species’ range. In 2000–2002, survey trawling, ultrasonic telemetry, benthos sampling, and hydrodynamic modeling were used to determine the summer movement patterns, home range size, and habitat use and selection of early juvenile Atlantic sturgeon in the St. Lawrence estuary. Sonic-tagged Atlantic sturgeon, assumed to be age 2 based on their size, used a global area estimated at 76 km2 that included two main core areas. Individual summer home range sizes varied from 1 to 8 km2, but core areas were generally smaller than 1 km2. The mean daily distance traveled ranged from 0.4 to 13.5 km/d and was significantly related to sturgeon size. Fish were located mostly in freshwater relatively close to the salt wedge boundary and far from the shore, intertidal zones, and islands. They mostly used the 6–10-m depth range relatively close to a channel, in areas with low bottom current velocities, and over silt–clay substrates. Salinity and distance from the salt wedge were the two most important variables explaining their habitat selection. Age-0 Atlantic sturgeon used similar depth ranges, bottom salinities, and current velocities, but occupied mainly sandy substrate. Management implications of these results are discussed in relation to the impact of dredging and sediment disposal operations in the St. Lawrence estuary.

Anadromous Sturgeons: Habitats, Threats, and Management

<em>Abstract</em>.—The anadromous Atlantic sturgeon <em>Acipenser oxyrinchus</em> once supported an important commercial fishery throughout its range (northern Florida, USA, to Labrador, Canada). All surviving populations are apparently depleted, presumably due to overfishing, pollution, and dam construction. A complete moratorium on the fishery has been established in U.S. waters. Unfortunately, population status is unknown for nearly all systems. Several 1994–2001 data sets from South Carolina rivers were examined for their potential in development of recruitment (year-class abundance) indices. Because Atlantic sturgeon often begin leaving their natal systems at age 2, after which they occupy other systems and then presumably return to natal areas at sexual maturity several years later, a valid river-specific abundance index must be based on either age0–1 or adult fish. Working with three former commercial Atlantic sturgeon fishermen to collect adults in two rivers during the spring and fall of 1998 resulted in collection of only 39 fish in 13 nominal age-classes. In another river, 2 years of monthly sampling with multipanel, anchored gill nets and otter trawls at eight stations throughout the estuary produced only 31 juvenile Atlantic sturgeon. Neither of these study designs provided adequate sample sizes. However, sampling in a clean-bottom section at the freshwater–brackish water interface of the Edisto River with a modified drift gill net produced large numbers of small Atlantic sturgeon. More than 3,000 juveniles have been collected and tagged since 1994. The 1,331 nominal age-1 Atlantic sturgeon that were captured displayed a distinct bimodal length frequency distribution, supporting the hypothesis that there are both spring and fall spawning events. A period of record drought impeded sampling efforts during a portion of this study. However, when flows and bottom configurations allow nets to be fished at the freshwater–brackish water interface, the gears and methods employed in the Edisto River can produce an age-1 Atlantic sturgeon catch per unit effort that is high enough to be used in estimating relative year-class strength.

Anadromous Sturgeons: Habitats, Threats, and Management

<em>Abstract</em>. — White sturgeon <em>Acipenser transmontanus</em> were once abundant in the lower Fraser River of British Columbia, but have been dramatically reduced by overfishing and habitat loss. Significant efforts have been put into collecting baseline abundance and demographic data over the past decade. However, much of the population biology is still unknown, thereby limiting the ability of managers to focus conservation efforts. For example, one of the pressing questions is the importance of slough habitats, which are used by young juveniles much more than older juveniles and adults. In the absence of direct estimates of birth and death rates, a valuable alternative is to infer these rates from data on population structure using demographic models. Here, we fit an age-based model for white sturgeon to the available length-frequency data from slough and river habitats. Our analysis of the parameterized model indicates that the white sturgeon population in the lower Fraser River was declining through the 1980s and into the 1990s. We estimate a growth rate in the range of λ = 0.90 to λ = 0.96, which corresponds to a 4–10% decrease in the population each year. This estimate agrees with an independent estimate of λ = 0.91 derived using only catch-per-unit-effort data on juvenile white sturgeon from a slough habitat. Sensitivity analysis of the fitted population model reveals that juvenile survival has the largest influence on population growth. Thus, we infer that improving juvenile survival in the slough habitats is key to conserving this white sturgeon population. We feel that observational and experimental studies that focus on the survival of young juveniles will have the largest impact on our understanding of white sturgeon population biology.

Anadromous Sturgeons: Habitats, Threats, and Management

<em>Abstract.</em>—The Gulf sturgeon <em>Acipenser oxyrinchus desotoi</em> is an anadromous species listed as threatened under the Endangered Species Act in 1991. We conducted a 3year tagging study to estimate population size, growth, mortality, and age composition for sturgeon in the Yellow River. Capture probabilities and population size were estimated using Program MARK and a Cormack-Jolly–Seber model. Total mortality of Gulf sturgeon was estimated using a Beverton–Holt mortality equation. Growth rate was determined from annuli on the leading edge of pectoral fin-ray. A total of 522 Gulf sturgeon captures were made, and 399 individual fish were tagged. The population estimates for the Gulf sturgeon over 3 years ranged from 500 to 911 fish. The age structure of the population suggests successful recruitment and a viable population. The total annual mortality estimate for Yellow River Gulf sturgeon was 11.9%. Growth rate for the Yellow River population was comparable to other populations of Gulf sturgeon. The Yellow River Gulf sturgeon population is a dynamic population based upon consistent age-classes as an indicator of successful recruitment, a large population size relative to most rivers where Gulf sturgeon are found, and estimates of mortality below the reported range for the species.

Anadromous Sturgeons: Habitats, Threats, and Management

<em>Abstract.</em>—Large-scale commercial fisheries for Atlantic sturgeon <em>Acipenser oxyrinchus</em> in the late 1880s eventually led to substantial reductions in the population size. The coastwide Atlantic sturgeon population of the United States has not recovered to the levels seen prior to the 1900s. A number of factors have contributed to the slow recovery or continued decline of Atlantic sturgeon populations, including continued commercial fishing and the targeting of females for caviar, bycatch in other fisheries, and changes in habitat due to dam construction and water quality degradation. The Atlantic States Marine Fisheries Commission (ASMFC) developed the first coastwide management plan for Atlantic sturgeon in 1990. In response to the shortcomings of that plan, the ASMFC applied new standards and the authority granted to it by the U.S. Congress to adopt a coastwide moratorium on all harvesting in 1998. A federal status review conducted in 1998 concluded that the continued existence of Atlantic sturgeon was not threatened given the situation at the time. Since then, monitoring programs have indicated varying levels of relative abundance in several water bodies along the Atlantic coast. The U.S. government is responsible for undertaking a status review to document any changes since the last review and determining whether those findings warrant a threatened or endangered listing for the species. The government’s findings may have far-reaching effects on many other Atlantic coastal fisheries.

Anadromous Sturgeons: Habitats, Threats, and Management

<em>Abstract.</em>— This paper describes a simulation study of reconnection options for white sturgeon <em>Acipenser transmontanus</em> subpopulations in adjacent river segments above and below CJ Strike Dam on the Snake River, Idaho, USA. In contrast to the downstream river segment, the upstream river segment is long and has areas that are suitable for spawning during normal and wet hydrologic conditions. We evaluated demographic and genetic consequences of upstream and downstream passage using different model assumptions about trashrack spacing and density-dependent effects on the spawning interval. Our genetic results predict that, although reconnection would introduce new alleles to the upstream subpopulation, it would also preserve alleles from the downstream subpopulation by propagating them in the larger subpopulation above the dam. Our demographic results predict that halving the space between trashracks would have large and unequivocal benefits, whereas the predicted effects of reconnection were smaller and more sensitive to model assumptions. Simulated upstream passage tended to benefit both subpopulations only in the absence of density-dependent limitation. In the presence of density dependence, the combination of halved trashrack spacing and upstream and downstream passage produced the best results. Narrower trashracks kept spawning adults in the upstream segment with spawning habitat, while allowing their progeny to migrate downstream. Screening appears to be the best option for such a species in this configuration of a long river segment acting as a demographic source above a short one acting as a demographic sink.

Anadromous Sturgeons: Habitats, Threats, and Management

<em>Abstract.</em> - Sea sturgeons are closely related anadromous fishes inhabiting both shores of the North Atlantic Ocean. They are classified in two species: the European sturgeon <em>Acipenser sturio</em> in Europe and the Atlantic sturgeon <em>A. oxyrinchus</em> in North America. The Atlantic sturgeon is further separated into two subspecies: Atlantic sturgeon (North American East Coast populations) <em>A. o. oxyrinchus</em> and Gulf sturgeon <em>A. o. desotoi. </em>Most recent studies of morphology and genetics support these classifications. Furthermore, they produced evidence for a trans-Atlantic colonization event during the early Middle Ages. Atlantic sturgeon colonized Baltic waters, founding a self-reproducing population before they became extinct due to anthropogenic reasons. Today, populations of Atlantic sturgeon are found along the Atlantic Coast from the St. Johns River, Florida to the St. Lawrence River, Quebec, whereas only one relict spawning population of European sturgeon still exists in the Gironde River, France. The evidence of a population of Atlantic sturgeon in Baltic waters requires a detailed comparison of both sea sturgeon species, describing differences and similarities, which may influence the ongoing restoration projects in Europe as well as concerning conservation efforts in North America. This article reviews similarities and differences in the fields of genetics, morphology, and ecological adaptation of European sturgeon and Atlantic sturgeon, concluding that, besides morphological and genetic differences, a wider range of spawning temperatures in Atlantic sturgeon is evident. This wider temperature adaptation may be a selective advantage under fast-changing climatic conditions, possibly the mechanism that enabled the species shift in the Baltic Sea during the Middle Ages.

Anadromous Sturgeons: Habitats, Threats, and Management

<em>Abstract.</em>—The short- and long-term impacts of the annual disposal of dredged sediment within Atlantic sturgeon <em>Acipenser oxyrinchus</em> habitat in the St. Lawrence estuary were assessed by simulating sediment transport over 10 years and by sampling sediment and macrobenthos on impacted and control radials. The model applied bed load transport functions embedded in a baroclinic hydrodynamic model. The path predicted by the model was validated at its half-way point (5 years) using multibeam sonar images of the seabed as well as qualitative and quantitative sediment analyses. Sediment took more than 1 year to completely clear the disposal site, indicating continuous sand drift along the predicted path. In the first 2 years, the sediment path increased rapidly in length (10 km), bypassing a field of sand dunes and following the deepest depressions (10 m) in the downstream direction. In the middle of its 10-year course (ca 12 km), the sediment track crossed the most important core area used by early juvenile Atlantic sturgeon. Afterward, sediment progression slowed and accumulated in a 15-mdeep channel between islands in the middle of the estuarine transition zone. The impact stations showed a significant reduction in the average biomass of tubificids, the most important food item of juvenile Atlantic sturgeon in the St. Lawrence. Conversely, the number and abundance of epi- and suprabenthic taxa increased. Model and field results suggest that sand drift generated from disposal operations reduces benthic productivity along the modeled path, including critical habitats of early juvenile Atlantic sturgeon. Sand accumulation from dredged spoil that drift in dead current areas are also likely to reduce juvenile sturgeon habitat quality.

Anadromous Sturgeons: Habitats, Threats, and Management

<em>Abstract.</em>—Declines in Atlantic sturgeon <em>Acipenser oxyrinchus</em> abundance in the early 1990s led the Atlantic States Marine Fisheries Commission (ASMFC) to prepare a mandatory fishery management plan. The principal management measures are fishery closure, bycatch assessment, and bycatch reduction in other ASMFC-managed fisheries (i.e., American shad <em>Alosa sapidissima</em>). To better understand Atlantic sturgeon geographic distribution and habitat use, as well as risk of bycatch, we examined offshore distribution of Atlantic sturgeon based on incidental captures in winter tagging cruises conducted off the coasts of Virginia and North Carolina, including in and near extensive sand shoals adjacent to Oregon Inlet and Cape Hatteras. From 1988 to 2006, 146 juvenile Atlantic sturgeon were captured by bottom trawling in depths from 9.1 to 21.3 m. Numbers of Atlantic sturgeon captured and tagged in a given year ranged from 0 (1993, 1995) to 29 (2006). Atlantic sturgeon were encountered in 4.2% of tows, with the percentage varying from 0 in 1993 and 1995 to 12.6% in 1988. Capture patterns suggested that Atlantic sturgeon were likely aggregating to some degree. Total lengths of captured Atlantic sturgeon ranged from 577 to 1,517 mm (mean of 967 mm), suggesting that most fish were juveniles. Limited tag returns and genetic data suggest that fish wintering off North Carolina constitute a mixed stock. Information about their distribution and habitat utilization should benefit fishery managers seeking to reduce bycatch and protect key habitats.

Anadromous Sturgeons: Habitats, Threats, and Management

<em>Abstract.</em>—The St. Lawrence estuarine transition zone (ETZ) harbors the only known concentrations of age-0 and early juveniles of the St. Lawrence Atlantic sturgeon and lake sturgeon populations. Past dredging and disposal operations conducted in the ETZ to deepen the navigation channel resulted in the creation of an extensive sand dune biotope near the juvenile sturgeon concentration areas. In order to characterize the dune biotope within a diversified set of biotopes in the ETZ, nine areas were selected for study, including two areas to cover the sand dune complex. The study objectives were (1) to identify the benthos assemblages of the ETZ and the main physical factors controlling them, (2) to measure the sampling areas’ biological characteristics and feeding potential for sturgeon, and (3) to compare the dune areas’ feeding potential with selected control areas. In 1999–2001, grab sampling was conducted at 141 stations to determine macrobenthos composition and sediment parameters. Depth, slope, and slope orientation were measured from multibeam sonar echosoundings. Salinity, current velocity, and tidal amplitude were provided by a hydrodynamic model of the ETZ. Benthos assemblages were determined using cluster analysis on taxon biomass. Four major assemblages were identified, all having Tubificidae as the dominant or subdominant taxon: zebra mussel <em>Dreissena polymorpha</em>, <em>Gammarus tigrinus</em>, Tubificidae, and <em>Capitella</em> sp. assemblages. A succession of the major assemblages was observed from the freshwater front to the upper mesohaline waters. Three minor assemblages, the Chironomidae, Physidae, and Cumacea, were concentrated in the upper oligohaline zone. Taxonomic richness was highest in areas with the lowest maximum salinity (0.0–0.5), and diversity was highest in areas with intermediate maximum salinities (0.5–2.0). The largest biomass values were found in areas with maximum salinities less than 0.5, in the zebra mussel assemblage. Controls and dune areas had similar macrobenthos richness and diversity, but dune areas had significantly lower densities and biomasses. Feeding potential for a given sturgeon life stage was measured as the sum of taxa biomasses standardized using the prey proportions in that life stage’s feeding regime. For age-0 Atlantic sturgeon and for all lake sturgeon life stages, all of which feed mostly on gammarids, the feeding potential of control and dune areas were not significantly different. For juvenile and subadult Atlantic sturgeon, which feed mostly on tubificids, the dune areas had a significantly lower feeding potential than the control areas. The lower feeding potential of the sand dune areas created by dredged sediment deposition is considered an important issue for the management of the St. Lawrence Atlantic sturgeon population.