Estimating survival of the shortfin mako Isurus oxyrinchus (Rafinesque) in the north-west Atlantic from tag-recapture data

Survival was estimated for shortfin mako Isurus oxyrinchus in the north-west Atlantic from tag-recapture data. The data used in this study were collected by the National Marine Fisheries Service Cooperative Shark Tagging Programme from 1962 to 2003. In total, 6309 shortfin mako sharks were tagged, of which 730 were recaptured. The high recapture rate of 11·6% for this species provided adequate recapture data to carry out survival analyses. Estimates of survival were generated with the computer software MARK, which provided a means for estimating parameters from tagged animals when they were recaptured. The results of several models are presented with various combinations of constant and time-specific survival and recovery rates. A parametric bootstrap and the median variance inflation factor ( ) approach were used to test the fit of the general model to the data. The estimated indicated a very good model fit. The models with time invariant survival rate had the most support from the data and no group or time period effects were found. Recovery rate ( f ) appeared to increase from 0·043 in the early years to 0·056 in the later years. The nominal survival rate of 0·59 year⁻¹ was adjusted with an estimated tag-shedding rate of 0·26 year⁻¹ to generate a final corrected annual survival estimate of 0·79 with a 95% CI of 0·71–0·87.     

Reproductive biology and population dynamics of the shortfin mako, Isurus oxyrinchus Rafinesque, 1810, off the southwest Portuguese coast, eastern North Atlantic

A total of 262 shortfin mako sharks, Isurus oxyrinchus, was sampled from the swordfish longliners operating in the eastern North Atlantic. Most were juveniles, with only 3.4% mature. Based on cohort analysis, average growth was determined as 61.1 cm year⁻¹ for the first year and 40.6 cm year⁻¹ for the second year. There was a marked seasonality in growth, with average monthly rates of 5.0 cm month⁻¹ in summer and 2.1 cm month⁻¹ in winter. Cohort analysis also indicated summer as the probable parturition season, with sharks close to birth size caught in May 2003 and July 2004. Length at maturity for males was estimated at 180‐cm fork length using the Schnute model. No females between 210 and 290‐cm fork length were caught, although this appears to be the interval where maturation occurs. Gear selectivity was considered as the probable cause for the low number of mature females sampled.     

Pugnose eels, Simenchelys parasiticus (Synaphobranchidae) from the heart of a shortfin mako, Isurus oxyrinchus (Lamnidae)

The body depth of crucian carp, Carassius carassius, increases in the presence of predator fish, thereby decreasing the vulnerability of crucian carp to predation. This phenotypic change is mediated by chemical signals, and is believed to result from a piscivorous diet of predators. We have shown that exposure to a piscivorous predator is insufficient to induce growth changes in crucian carp, since water from northern pike, Esox lucius, fed Arctic charr, Salvelinus alpinus, does not induce a change in crucian carp morphology, while water from pike fed crucian carp does. The determining factor is a chemical signal from the skin of crucian carp, as demonstrated by exposure to skin extracts from conspecifics. We suggest that alarm substances from conspecifics, expressing primer pheromone effects, are the most likely candidates for induction of the phenotypical changes.     

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.     

Validity of an external sex determination method in Atlantic Sturgeon (Acipenser oxyrinchus oxyrinchus)

The validity of the shape of the urogenital opening was tested as a predicable means to determine the sex of Atlantic sturgeon captured in the Saco River estuary, Maine. Evaluation of 121 individuals ranging in size from 107 to 182 cm fork length were compared to non‐lethal radioimmunoassay determined sex data previously examined for these individuals within this estuary. The results suggested that using the shape of the urogenital opening is not a reliable means to determine sex as only 51% of Atlantic sturgeon were correctly identified as female. Additionally, there was no significant difference in correctness relative to 10 cm fork length size classes or sampling month.     

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.     

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.

     

Oceanic Distribution,Behaviour, and a Winter Aggregation Area of Adult Atlantic Sturgeon,Acipenser oxyrinchus oxyrinchus,in the Bay of Fundy,Canada

Seasonal distribution of adult Atlantic sturgeon was examined using pop-up satellite archival tags (PSATs) and ultrasonic transmitters deployed in the Saint John River, New Brunswick, Canada. Seven MK10 PSATs programmed for release in June 2012 and seven MiniPAT PSATs programmed for release in February and April 2013 were deployed in August 2011 and 2012, respectively. Eleven of 14 PSATs surfaced and transmitted depth and temperature data archived for the duration of their deployment (121–302 days). Among these eleven PSATs, five were recovered and 15-sec archival data was downloaded. Following exit from the Saint John River in the fall, tagged fish occupied a mean monthly depth of 76.3–81.6 m at temperatures as low as 4.9˚C throughout the winter before returning to shallower areas in the spring. The majority of ultrasonic detections occurred in the Bay of Fundy, but fish were detected as far as Riviere Saint-Jean, Quebec, approximately 1500 km from the Bay of Fundy (representing long-distance migratory rates of up to 44 km/day). All PSATs were first detected in the Bay of Fundy. Tags that released in February and April were found 5–21 km offshore of the Saint John Harbour, while tags that released in June were first detected in near shore areas throughout the Bay of Fundy. The substrate at winter tag release locations (estimated from backward numerical particle-tracking experiments) consisted primarily of moraines and postglacial mud substrate with low backscatter strength, indicative of soft or smooth seabed. Based on the proximity of winter tag release locations, the consistent depths observed between fish, and previous research, it is suspected that a winter aggregation exists in the Bay of Fundy. This study expands the understanding of the marine distribution and range of Atlantic sturgeon on the east coast of Canada.     

Age and growth of the small red scorpionfish, Scorpaena notata Rafinesque, 1810, based on whole and sectioned otolith readings

Age and growth of Scorpaena notata from the northern Adriatic Sea were investigated by annual growth increment counts (annuli). Overall, age and growth were estimated from 538 specimens of S. notata ranging between 47 and 199 mm TL. No clear sexual dimorphism in size was observed. Annual deposition of annuli and location of the first annulus have been validated by edge analysis and daily growth increment counts, respectively. The estimated age range was between 0?C16 years for female and 0?C14 years for males. The estimated values of Von Bertalanffy asymptotic length $ {{\text{L}}_{\infty }} $ (cm) and k (years^?1) were 16.3 and 0.46 for males and 15.6 and 0.35 for females. Thus, males of S. notata appeared to attain a slightly larger size at faster rate than females. The growth performance index ranged between 1.9 and 2.1, which is in the middle of the range observed in other scorpaenids. Comparing ageing data of S. notata with other Mediterranean scorpaenids, a direct relationship between fish size and growth performance was observed.     

New insights on the trophic ecology of blue (Prionace glauca) and shortfin mako sharks (Isurus oxyrinchus) from the oceanic eastern South Pacific

The blue shark (Prionace glauca) and the shortfin mako shark (Isurus oxyrinchus) are two large and highly migratory sharks distributed in most oceans. Although they are often caught in the south Pacific Ocean long-line fisheries, their trophic ecology is poorly understood. Stable isotopes with Bayesian mixing and dependence concentration models were performed to determine the diet and trophic differences between the two species in the South-eastern Pacific Ocean. According to the mixing models, fishes are the most important prey of these sharks. Dolphin calves and remains were found in the stomachs of both species, which represents a novel finding in trophic ecology of South Pacific sharks. Intra-specific differences were found in P. glauca, but not in specimens of I. oxyrinchus. The two sharks showed a high degree of diet overlap (73%), primarily over mackerel and dolphin carcasses. Our results indicate that blue and shortfin mako sharks have a generalist feeding strategy in the eastern Pacific Ocean, with a strong preference for teleost fishes and also for dolphin carcasses. Therefore, trophic studies are useful to understand energy flow through the food web, and the trophic position of key species.     

Spatial attachment-site preferences of macroectoparasites on Atlantic sturgeons Acipenser oxyrinchus in Minas Basin, Bay of Fundy, Canada

Although parasite habitat preference is well studied, it is rarely rigorously evaluated statistically because of many zero intensities. Attachment-site preference and intensities of 2 macroectoparasite species ( Caligus elongatus and Calliobdella vivida ) of Atlantic sturgeon, Acipenser oxyrinchus Mitchill, in Minas Basin, Bay of Fundy, Canada, were characterized with the use of zero-inflated negative binomial statistical models that included a fork-length offset to control for body size. Three other parasites were encountered, sometimes in high numbers on various body sites, but too few counts overall prevented construction of meaningful statistical models. Of 26 sturgeons, prevalence of (1) C. elongatus (Copepoda) was 85%, mainly on caudal fins and nonfin body sites; (2) C. vivida (Hirudinea) was 81%, mainly on the pelvic and pectoral fins, and dorsal and ventral-lateral body sites; (3) Dichelesthium oblongum (Copepoda) was 31% within the gills or burrowed into the musculature at the base of fins; (4) Argulus stizostethii (Crustacea: Branchiura) was 8%; and (5) Nitzschia sturionis (Monogenea) was 12%. Only D. oblongum was associated with visible damage, mainly as lesions on gills and soft tissues. Characterizing parasite prevalences within the Bay of Fundy is important because some parasites affect fish health and population biology.