Atlantic sturgeon Acipenser oxyrinchus surfacing behaviour

Atlantic sturgeon Acipenser oxyrinchus surfacing behaviour was investigated in Minas Basin (45° 20′ N; 64° 00′ W) and the Bay of Fundy with pop‐up satellite archival tags (MiniPAT) measuring physical variables (pressure, temperature, light ) . Of six tags deployed during June and July, five provided pop‐up locations and two were recovered after c. 4 months. Analysis of recovered archival data revealed that the frequency of surfacing events was highest (78·9%) when A. oxyrinchus were in Minas Basin at depths <10 m. Surfacing frequency decreased substantially when fish migrated into greater depths of the Bay of Fundy (>40 m). The tidal cycle in Minas Basin had a significant relationship to surfacing frequency, with the most surfacing events (49·5%) occurring on the flood tide, from mid‐ to high‐tide. Surfacing events ranged from 0–12 a day and the maximum number occurred between 2300 and 0300 hours. Maximum surfacing ascent speeds ranged from 0·50 to 4·17 m s^?1 and maximum descent speeds ranged from 0·17 to 3·17 m s^?1. Buoyancy control, by gulping air to inflate the gas bladder, is proposed as the main reason for surfacing behaviour in A. oxyrinchus.     

Status and management of Atlantic sturgeon, Acipenser oxyrinchus, in North America

The Atlantic sturgeon, Acipenser oxyrinchus, consists of two subspecies distributed along the Atlantic coast of North America from Labrador to the east coast of Florida (Atlantic sturgeon subspecies – A. o. oxyrinchus) and along the Gulf of Mexico from Florida Bay, Florida to the mouth of the Mississippi River (Gulf sturgeon subspecies – A. o. desotoi). The species has been exploited throughout its range with landings peaking around the turn of the 20th century followed by drastic declines shortly thereafter. During recent years, landings in Canadian waters have increased substantially (approximately 129 metric tons in 1993) while in the United States landings are more controlled or prohibited (approximately 22–24 metric tons in 1993). Recently, the Atlantic States Marine Fisheries Commission developed a Fishery management plan for Atlantic sturgeon, and the United States Fish & Wildlife Service and Gulf States Marine Fisheries Commission drafted a Gulf Sturgeon Recovery/Management Plan. Fishery managers in Canada are in the process of establishing more stringent fishery regulations for sturgeon. Thus, the impact on populations due to harvesting should be substantially reduced. Current research focus includes: life history and population status studies, stock delineation, and development of culture and stock enhancement techniques. Implementation of the findings of such studies may be helpful in the restoration of depleted stocks.     

Population origin of Atlantic sturgeon Acipenser oxyrinchus oxyrinchus by‐catch in U.S. Atlantic coast fisheries

Microsatellite DNA and mitochondrial DNA control‐region sequence analyses were used to determine the population and distinct population segment (DPS) origin of 173 Atlantic sturgeon Acipenser oxyrinchus oxyrinchus encountered from the Gulf of Maine to Cape Hatteras, North Carolina, in NOAA's Northeast Fisheries Observer Program. It was found that the Hudson River was by far the greatest contributor to this coastal by‐catch, with 42·2–46·3% of specimens originating there. Generally, specimens represented the geographic province of the river in which they were spawned, but some specimens, particularly those originating in the South Atlantic DPS, moved to great distances. Genetic mixed‐stock analyses provide an accurate approach to determine the DPS and population origin of A. o. oxyrinchus by‐catch in coastal waters, but most informative management requires that these results be partitioned by locale, season, target fishery and gear type.     

Changes in age composition and growth characteristics of Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus) over 400 years

Populations of sturgeon (Acipenseridae) have experienced global declines, and in some cases extirpation, during the past century. In the current era of climate change and over-harvesting of fishery resources, climate models, based on uncertain boundary conditions, are being used to predict future effects on the Earth''s biota. A collection of approximately 400-year-old Atlantic sturgeon spines from a midden in colonial Jamestown, VA, USA, allowed us to compare the age structure and growth rate for a pre-industrial population during a ‘mini-ice age’ with samples collected from the modern population in the same reach of the James River. Compared with modern fish, the colonial population was characterized by larger and older individuals and exhibited significantly slower growth rates, which were comparable with modern populations at higher latitudes of North America. These results may relate to higher population densities and/or colder water temperatures during colonial times.     

Microsatellite DNA variation in Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus) and cross-species amplification in the Acipenseridae

Overharvest and habitat alteration have led toa collapse of most commercial Atlantic sturgeon(Acipenser oxyrinchus oxyrinchus)fisheries while pushing the species to rarityor extirpation in most of its historical range. A biologically sound conservation program forthis species requires knowledge of its geneticdiversity and of the evolutionary relationshipsamong geographic populations. To address theseresearch needs, six microsatellite loci wereisolated from A. o. oxyrinchus. Pedigreeanalysis suggested that all are inherited in acodominant Mendelian pattern. The six lociwere tested in ten additional sturgeon speciesfrom three genera and three apparent ploidylevels (4n, 8n, 16n). Approximately 70% ofsuccessful locus-species amplifications werepolymorphic. Polysomy was observed most oftenin 8n and 16n species. Genetic diversity andpopulation structure of A. o. oxyrinchuswere assayed using three polymorphic Aoxmarkers and four markers developed from lakesturgeon (A. fulvescens). A. o.oxyrinchus were sampled from the AltamahaRiver, Georgia, USA north to the St. LawrenceRiver, Quebec, Canada. Gulf sturgeon, A.o. desotoi, were sampled from the SuwanneeRiver, Florida, USA, to assess differentiationbetween the subspecies. Seventy-seven alleleswere observed to segregate into uniquemultilocus genotypes for each of the 392individuals assayed. Mean diversity wasgreatest in the Chesapeake Bay (9.7 alleles perlocus) and Delaware River (7.4 alleles perlocus) collections, and lowest in the St.Lawrence River (4.6 alleles per locus). Meanheterozygosity across seven loci ranged from44.3% (St. Lawrence River) to 62.6% (Altamaha River). Significant allelic heterogeneity wasobserved in 82% of pairwise comparisons aswell as a global test (p < 0.0001) for A.o. oxyrinchus collections. Genetic distancesuggests the presence of at least sixsubpopulations in A. o. oxyrinchus: St.Lawrence River, St. John River, Hudson River,Delaware River, Albemarle Sound, and AltamahaRiver. Genetic and geographic distances werepositively correlated (r = 0.57, p < 0.03) amongA. o. oxyrinchus, suggesting isolation bydistance and philopatry. Hierarchical genediversity analysis indicated significantgenetic population structure at every level. Maximum likelihood assignment tests correctlyassigned individual fish to collection with ahigh rate of success (mean = 87.5%); this andother lines of evidence indicated that theChesapeake Bay collection represents a mixedpopulation of sub-adult sturgeon from northernand southern Atlantic coast populations. Population structure was correlated with thatsuggested by earlier mitochondrial (mt) DNAanalyses. Significant diversity was observedbetween two Canadian populations from whichonly a single mtDNA haplotype has beenreported.     

Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus Mitchill, 1815) early life stage consumption evidenced by high-throughput DNA sequencing

Predation on early life stages of Atlantic Sturgeon Acipenser oxyrinchus oxyrinchus (ATS) may be a constraint to species recovery. Due to the difficulty in assessing consumption of early life stages with traditional diet analysis methods, we pursued an alternative DNA-based approach. We extracted total gut content from gastrointestinal tracts of 23 fish species (593 samples) within the tidal-fresh Pamunkey River fish assemblage collected from known ATS spawning grounds during prime spawning periods (September-October) in 2016. High-throughput DNA sequencing was used to amplify two markers for each sample: mitochondrial cytochrome c oxidase I (COI) and nuclear 18S ribosomal RNA gene. DNA sequences were compared to custom, SILVA 132, and NCBI databases. Results showed presence of ATS DNA in 22 samples (4%) across multiple native and nonnative tidal-fresh fishes. The highest percentage of consumption occurred in samples from Common Carp Cyprinus carpio (11.5) and Striped Bass Morone saxatilis (12.5). Six percent of Blue Catfish Ictalurus furcatus samples had target DNA. Considering there were no bony structures or tissue resembling ATS during morphological evaluation, DNA detections were likely from partially-fully digested eggs or early developing days-old larvae. High-throughput sequencing is capable of detecting consumption of ATS.     

Stock composition of Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus) encountered in marine and estuarine environments on the U.S. Atlantic Coast

Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus) is a large, anadromous fish native to the Atlantic Coast of North America. Although this species once supported important fisheries, centuries of exploitation and habitat degradation have resulted in dramatic declines, presumed extirpation in some rivers, and ultimately listing under the U.S. Endangered Species Act (ESA). Under the ESA, Atlantic sturgeon are listed as five separate Distinct Population Segments (DPSs), which form the basis for federal management. Despite state and federal protections Atlantic sturgeon still face significant threats to their recovery, including fisheries bycatch mortality, marine construction, dredging, dams, and vessel strikes. However, because subadult and adult Atlantic sturgeon migrate extensively across estuarine and marine environments and frequently form mixed-stock aggregations in non-natal habitats, it can be difficult to determine how these threats impact specific populations and DPSs. To better understand ontogenetic shifts in habitat use and stock-specific exposure to anthropogenic threats, we performed a mixed-stock analysis of 1704 Atlantic sturgeon encountered across the U.S. Atlantic Coast. Collections made north of Cape Cod, MA and south of Cape Hatteras, NC were dominated by individuals from regional stocks; however, we found extensive stock mixing in the mid-Atlantic region, particularly in coastal environments where individuals from all five DPSs were commonly observed. Subadults and adults that were encountered in offshore environments had moved, on average, 277 km from their natal source; however, 23% were sampled over 500 km from their natal river suggesting long-distance movements are relatively common in these age classes. Overall, our work highlights that Atlantic sturgeon populations are vulnerable to threats over vast areas and emphasizes the need for continued genetic monitoring to track recovery progress.

     

Laboratory test of suspended sediment effects on short‐term survival and swimming performance of juvenile Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus,Mitchill, 1815)

Tested was the hypothesis that juvenile Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus) would exhibit no response in short‐term survival or swimming performance when exposed to varying concentrations of suspended sediment simulating dredge plumes in waterways where this species may be impacted by dredging operations. Sediment collected from Savannah Harbor, South Carolina, USA was used to simulate a worst‐case scenario. Juvenile sturgeon were contained for a 3‐day period in flow‐through aquaria, with limited opportunity for movement, in sediment of varying concentrations (100, 250 and 500 mg L^?1 total suspended solids [TSS]) mimicking prolonged exposure to suspended sediment plumes near an operating dredge. Of the 90 fish exposed, 86 (96%) survived the test. Of the four fish that died, one was exposed to 250 TSS and three to 500 TSS. Swimming performance results indicated that nearly all fish were positively rheotactic. Critical swim speeds (U_crits) were moderate, whether measured as absolute values (21–31 cm s^?1) or as relative values (1.4–2.1 body lengths s^?1), with no significant differences among treatments (F < 0.83, P ≥ 0.4874). Behavior was dominated by contact‐based locomotion and station‐holding. Absence of substantial or significant immediate effects on survival and swimming performance suggest that impacts of sediment plumes in nature, where fish have freedom of movement and the power to escape rapidly, are minimal.     

Evidence of autumn spawning in Suwannee River Gulf sturgeon,Acipenser oxyrinchus desotoi (Vladykov, 1955)

Evidence of autumn spawning of Gulf sturgeon Acipenser oxyrinchus desotoi in the Suwannee River, Florida, was compiled from multiple investigations between 1986 and 2008. Gulf sturgeon are known from egg collections to spawn in the springtime months following immigration into rivers. Evidence of autumn spawning includes multiple captures of sturgeon in September through early November that were ripe (late‐development ova; motile sperm) or exhibited just‐spawned characteristics, telemetry of fish that made >175 river kilometer upstream excursions to the spawning grounds in September–October, and the capture of a 9.3 cm TL age‐0 Gulf sturgeon on 29 November 2000 (which would have been spawned in late September 2000). Analysis of age‐at‐length data indicates that ca. 20% of the Suwannee River Gulf sturgeon population may be attributable to autumn spawning. However, with the very low sampling effort expended, eggs or early life stages have not yet been captured in the autumn, which would be the conclusive proof of autumn spawning. More sampling, and sampling at previously unknown sites frequented by acoustic telemetry fish, would be required to find eggs.     

Relative physiological effects of laparoscopic surgery and anesthesia with tricaine methanesulfonate (MS‐222) in Atlantic sturgeon Acipenser oxyrinchus oxyrinchus

Laparoscopy is a reliable, minimally‐invasive technique to obtain reproductive information from wild and captive sturgeon. While generally considered safe, the physiological consequences of laparoscopy in sturgeon are unknown. Therefore clinical pathology changes in juvenile, Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus) following experimental laparoscopy at 10 and 22°C were described. Control fish were anesthetized with MS‐222 according to the same protocols as surgical fish, but were not incised. Surgical procedures did not affect heart and ventilation rates, signs of stress (skin redness) or time to recover from anesthesia in comparison to control fish. Anesthesia with MS‐222 produced a transient (by 1 h) hemo‐concentration (elevated protein and electrolytes), erythrocyte swelling (increased PCV and MCV) and stress response (elevated cortisol and glucose); and a delayed (by 24 h) increase in RBC, leukopenia and increased N : L ratio. Surgical procedures resulted in a delayed (by 24 h) decrease in plasma proteins, electrolytes, RBC and PCV relative to control fish, which may have resulted from surgically‐induced hemorrhage. Plasma enzyme activities increased in response to anesthesia and surgery and may indicate general stress and tissue damage. Anesthesia had a greater effect on blood value response than surgery, and the proportion of effect increased with temperature as MS‐222 potency and toxicity increases with water temperature. Repeated handling and blood draws within 24 h resulted in a 7% increase in cortisol, 10–14% increase in CK and 9–11% increase in LDH values. Except for plasma enzyme activities, blood values of all fish recovered within 1 week following anesthesia and surgeries. Relative experience of surgeons had no effect on hematology and biochemistry of fish, but healing rates of incisions were improved with better suture technique. Results of this study conclude that the physiological effects of laparoscopy are largely related to the anesthetic, MS‐222, and are generally mild and short‐lived. Improvements in laparoscopic technique might be gained by exploring alternate anesthetic protocols with faster induction and recovery times and reduced physiological effects.     

Ontogenetic Behavior and Migration of Atlantic Sturgeon, Acipenser oxyrinchus oxyrinchus, and Shortnose Sturgeon, A. brevirostrum, with Notes on Social Behavior

Ontogenetic behavior of Hudson River Atlantic sturgeon and Connecticut River shortnose sturgeon early life intervals were similar during laboratory observations. After hatching, free embryos were photonegative and sought cover. When embryos developed into larvae, fish left cover, were photopositive, and initiated downstream migration. Free embryos may remain at the spawning site instead of migrating downstream because the risk of predation at spawning sites is low. The two species are sympatric, but not closely related, so the similarities in innate behaviors suggest common adaptations, not phylogenetic relationship. Atlantic sturgeon migrated downstream for 12 days (peak, first 6 days), shortnose sturgeon migrated for 3 days, and year-0 juveniles of both species did not resume downstream migration. Short or long migrations of larvae may reflect different styles related to the total migratory distance from spawning sites to juvenile rearing areas. Atlantic sturgeon need to move a short distance to reach rearing areas and they had a long 1-step migration of 6–12 days. In contrast, shortnose sturgeon need to move a long distance to reach all rearing areas. This may be accomplished by a 2-step migration, of which the brief migration of larvae is only the first step. Early migrant Atlantic sturgeon were nocturnal, while late migrants were diurnal, and shortnose sturgeon were diurnal. These diel differences may also be adaptations for long (Atlantic sturgeon) or short (shortnose sturgeon) migrations. Cultured shortnose sturgeon, and possibly Atlantic sturgeon, have a dominance hierarchy with large fish dominant when competing for limited foraging space. Social behavior may be more important in the life history of wild sturgeons than is generally recognized.     

Migration patterns and survival of stocked Atlantic sturgeon (Acipenser oxyrinchus Mitchill, 1815) in Nemunas Basin,Baltic Sea

The restoration of the Atlantic sturgeon Acipenser oxyrinchus in Lithuania started in 2011. Two rivers (Neris and ?ventoji) were chosen for reintroduction in the Nemunas river basin based on historical data and ecological conditions. Since 2011, more than 116 thousand of sturgeon juveniles have been stocked in Lithuania. In order to achieve successful population restoration, it is vital to evaluate the efficiency of performed artificial stocking. Analysis of post‐stocking survival, migration patterns, predation pressure, fisheries‐related mortality and possible aggregation zones are all important to understand the main threats and prepare possible mitigation measures necessary for population establishment during the initial phase of restoration. Therefore, conventional and radio tagging studies were performed during the initial restoration phase in Lithuania. Atlantic sturgeon downstream migration was divergent in summer and appeared to have no significant associations with fish size, river quality or hydrophysical parameters, but in autumn migration speed and even survival in small rivers seemed to depend on flow velocity and water discharge. The distribution of recaptured tagged juveniles coincided with dominant currents in the Curonian Lagoon and Baltic Sea. Some tagged specimens migrated long distances, with two sturgeons from Lithuania being caught in Estonian and Finnish territorial waters at up to 800 km from their release site. Tag recapture analysis and mortality rate of tagged fish clearly indicate that annual fisheries‐related mortality exceeds the required threshold of 5% for population establishment. High survival in rivers indicates that the main threat for a successful sturgeon reintroduction program is commercial fishing in the Curonian Lagoon and Baltic Sea coastal zone.     

Novel microsatellite markers for Atlantic sturgeon (Acipenser oxyrinchus) population delineation and broodstock management

Twenty‐one new disomic, polymorphic microsatellite DNA markers were isolated in Atlantic sturgeon, Acipenser oxyrinchus oxyrinchus. These markers yielded a total of 220 alleles in a survey of 16 fish; two to 21 alleles/locus were observed. Each locus segregated in a Mendelian fashion when tested in a family, and a set of 14 of the loci distinguished between siblings and half‐siblings. Average observed heterozygosity ranged from 18.8 to 100.0%, and no linkage disequilibrium was detected. These loci should detect sufficient genetic diversity to allow kinship analysis for broodstock management, gene marking for stocking assessment and life history studies, and delineation of fine‐scale population structure.     

Genetic mixed-stock analysis of Atlantic sturgeon Acipenser oxyrinchus oxyrinchus in a heavily exploited marine habitat indicates the need for routine genetic monitoring

Although a previous genetic mixed-stock analysis (gMSA) conducted in the early 1990s showed that marine-captured New York Bight Atlantic sturgeon Acipenser oxyrinchus oxyrinchus almost exclusively originated from the Hudson River, fish from southern U.S. rivers were well represented within this contemporary sample (n = 364 fish), at least during the autumn. Widely distributed spawning stocks are therefore exposed to heavy fishing activity and habitat degradation in this relatively small area, illustrating the need for spatial management across multiple management jurisdictions and routine gMSA to account for temporal change.     

Structure and management of an exploited population of Atlantic sturgeon (Acipenser oxyrinchus) in the St. Lawrence Estuary, Québec, Canada

The Atlantic sturgeon ( Acipenser oxyrinchus ) has been exploited commercially in the upper estuary of the SL Lawrence River right up until the present day. Statistics on annual landings from 20 to 401 between 1940 and 1966, and an almost total absence between 1967 and 1975, were followed by a significant capture increase, which reached 120 t in 1993.
Analysis of captures has shown that the median age of sturgeon caught has gone down from 11 to 6 years-old in recent years, that the length of the fish (LF) varied between 50 cm and 170 cm, and that the last significant cohorts supporting the fishery come from 1982, 1986 and 1988. In 1994, 2% to 4% of captures were larger than 150 cm. No fish in reproductive state, even among the largest individuals, was observed among the captures.
Since 1995, we have adopted various regulations in order to protect the largest sturgeons, and to reduce the annual landings to 60 t in 1998. Monitoring of captures, conducted in collaboration with commercial fishermen, has enabled us to measure the effects of these new fishing regulations, which include fishing effort control, reduction of the season, size limits for captures, and imposition of a quota. These measures have allowed us to partially reach our objective, since landings in 1997 were reduced to 661.
The absence of sexually mature adults in captures has always raised the question as to their abundance and the areas which they use. Current studies are demonstrating that the sexually mature adults use the deepest parts of the estuary, sectors which are not exploited in a sustained way by fishermen. Studies are in progress to locate spawning grounds and early juvenile habitat. Exploitation of this population will continue to be closely monitored.     

Atlantic sturgeon Acipenser oxyrinchus Mitchill, 1815 seasonal marine depth and temperature occupancy and movement in the Bay of Fundy

Pop‐up satellite archival tags were used to collect fisheries‐independent data that characterized the seasonal habitat occupancy and movement of Atlantic sturgeon Acipenser oxyrinchus Mitchill, 1815 in the Bay of Fundy (BoF). Atlantic sturgeon from Canadian and United States stocks aggregate annually for feeding in Minas Basin, inner BoF (45.28N, 64.18W), during May to September but depart to other locations from October to April. Sixteen PSAT tags were applied to sturgeon ranging from 152 to 203 cm total length captured and released in Minas Basin during May to August. Ten of the tags were reported after 1 year at large and provided pop‐off locations. Seven were recovered with archived data, or provided transmitted data sets, which were analyzed for depth and temperature occupancy from June 2011 to August 2013. During June to August while in the Minas Basin the sturgeon spent >90% of time at depths <10 m and temperatures of 16–22°C. Departure from Minas Basin through the Minas Passage was in September and October, when depth occupancy varied from <10 to 120 m. From November to April sturgeon were in the outer BoF, where mean depth occupancies ranged from 40 to 100 m at mean temperatures of 3–14°C. Deepest mean depth occupancy of 60 to 90 m was recorded during December 2011 and 2012, and coldest mean temperature occupancy of 0–4°C in March 2011 and 2012. During April and May mean depth and temperature occupancy ranges shallowed from 40 to <10 m and increased from 4 to 15°C, respectively. Tag pop‐off locations indicated that sturgeon spent the winter season in the outer BoF but by June had either migrated back to the Minas Basin or off the mouth of the Saint John River, a known spawning location.