John D. Neilson

Natal Homing and Connectivity in Atlantic Bluefin Tuna Populations

Atlantic bluefin tuna populations are in steep decline, and an improved understanding of connectivity between individuals from eastern (Mediterranean Sea) and western (Gulf of Mexico) spawning areas is needed to manage remaining fisheries. Chemical signatures in the otoliths of yearlings from regional nurseries were distinct and served as natural tags to assess natal homing and mixing. Adults showed high rates of natal homing to both eastern and western spawning areas. Trans-Atlantic movement (east to west) was significant and size-dependent, with individuals of Mediterranean origin mixing with the western population in the U.S. Atlantic. The largest (oldest) bluefin tuna collected near the northern extent of their range in North American waters were almost exclusively of western origin, indicating that this region represents critical habitat for the western population.

A validated description of age and growth of western Atlantic bluefin tuna (Thunnus thynnus)

Current stock assessments of western Atlantic bluefin tuna (Thunnus thynnus) use age-structured population analyses, but the age assignment in the population model is made using an age–length relationship derived from mark and recapture studies largely completed during the 1970s. In our study, the deposition of bomb radiocarbon was used as a dated mark to validate age inferences of bluefin tuna and to compare the validated ages with those predicted from the age–length relationship. The results support the view that the age–length relationship currently in use for the assessment overestimates growth rate and the ultimate size of the fish. These findings have implications for the estimation of stock productivity and may negatively impact the rebuilding schedules established by fisheries managers.

Investigations of Horizontal Movements of Atlantic Swordfish Using Pop-up Satellite Archival Tags

The long-distance horizontal movements of swordfish tagged in Canadian waters are described, using information obtained from pop-up satellite archival tags. Some of the satellite tags remained attached to the fish for up to 411 days, among the longest periods of attachment of pop-up satellite tags reported for any fish species. The results to date challenge the assumption employed in current stock assessments that swordfish move freely from the western North Atlantic to the eastern North Atlantic, as no such movement has been found. The results also demonstrate a consistent pattern of movement with residence in temperate waters from June to October, followed by migration to the south into the Caribbean Sea, with fish remaining there until April. Tagged swordfish returned to temperate waters by June, with evidence of precise homing to feeding areas.

Natal origin of Atlantic bluefin tuna (Thunnus thynnus) from Canadian waters based on otolith δ13C and δ18O

Increased knowledge of stock mixing and migration of Atlantic bluefin tuna (Thunnus thynnus) is required to properly manage and conserve declining populations. Here, we predicted the nursery origin of giant bluefin tuna (n = 224) present in samples from Canadian waters using stable δ13C and δ18O isotopes in otoliths. The isotopic composition of milled otolith cores (corresponding to the first year of life) of giant bluefin tuna from three decades (1970s, 1980s, 2000s) and three regions within or adjacent to the Gulf of St. Lawrence was compared with otolith δ13C and δ18O of yearling bluefin tuna collected from eastern (Mediterranean Sea – eastern Atlantic, n = 136) and western (western Atlantic, n = 103) nurseries. Maximum likelihood estimates indicated that greater than 99% of bluefin tuna in our Canadian samples originated from the western nursery. No significant differences in estimates of origin for bluefin tuna were detected among decades or among regions, suggesting little to no mixing of eastern an...

The diel-vertical distribution and characteristics of pre-spawning aggregations of pollock (Pollachius virens) as inferred from hydroacoustic observations: the implications for survey design

The characteristics of pollock (the synonymous European common name is saithe) pre-spawning aggregations were described at two locations with contrasting bathymetric features on the Scotian Shelf, off the Canadian Maritimes. The data were collected using a split-beam echosounder onboard a research vessel, augmented with periodic, bottom-trawl samples. Pollock form aggregations each fall that persist at the same location over time. Such aggregations appeared to be associated with spawning. Hydroacoustic information indicates that pollock become more densely aggregated at night. This could reflect movement away from the study area during the day, or changes in the proportion of pollock in the acoustic dead zone over the 24-h period. The hydroacoustic information indicates that while pollock can occur up to 30 m off bottom, the greatest proportion remains within 1-5 m off bottom during both day and night. The length composition of the pollock aggregations differed between the two sites, with larger fish found at the site further offshore. Within an aggregation, there was spatial heterogeneity with respect to fish size, with larger fish found primarily within the core area of aggregation as shown by the hydroacoustics. An appropriate survey design for obtaining an index of abundance for pollock would reflect both the contagious (patchy) distribution as they prepare to spawn, and the diel differences in the availability of the fish to the hydroacoustic-sampling gear.

The Recovery of Atlantic Swordfish: The Comparative Roles of the Regional Fisheries Management Organization and Species Biology

Regional fisheries management organizations (RFMO) are often criticized for ineffectual management of high-seas fisheries resources. However, in the case of the two Atlantic swordfish stocks occurring in the North and South Atlantic, those stocks have rebuilt to the BMSY objective of the responsible RFMO, the International Commission for the Conservation of Atlantic Tunas (ICCAT). The relative contribution of the management actions of the RFMO and biological characteristics of swordfish stocks are evaluated in relation to the recovery of the stocks. It is concluded that while swordfish have characteristics that promote stock resilience (including relatively fast growth, and spatially- and temporally-dispersed spawning), positive management actions combined with a period of relatively good recruitment were essential to achieve the rebuilt outcome. The challenges that the RFMO faces to maintain the stocks in the rebuilt condition are described, and some possible additional measures discussed.

Determination of Lmax for Atlantic Bluefin Tuna, Thunnus thynnus (L.), from Meta-Analysis of Published and Available Biometric Data

A meta-analysis of the straight fork lengths (herewith abbreviated as L) of 2,458,028 Atlantic bluefin tuna, Thunnus thynnus (L.), taken from 224 scientific publications and unpublished L data from scientific organizations and fishing companies spanning most of the known Atlantic and Mediterranean Atlantic bluefin tuna fisheries dating from 1605 to 2011, give L values ranging from L min = 20 cm and L max = 330 cm. The results indicate that the parameter L ∞ = 318.85 cm of the growth equation used by ICCATs Standing Committee on Research and Statistics Atlantic bluefin tuna assessment group for the eastern stock (Lt = 318.85 [1 – e−0.093 (t + 0.97)]) lies within the confidence limits of the maximum Ls presented in the study: L max = 319.93 ± 11.3 cm, confirming that this equation perfectly fits the biology of the growth of this species. These conclusions are also valid for the equation for the western stock (Lt = 314.90 [1 – e−0.089 (t +1.13)]). The ICCAT Atlantic bluefin tuna database contains numerous records of Atlantic bluefin tuna L outside the biological feasibility, and solutions are provided to recognize and remove these outliers based on the application of fixed values of Fultons condition factor (K) between 1.4 and 2.6 and appropriate L-W relationships to correct this situation in the future.

Seasonal distributions and migrations of Northwest Atlantic swordfish: inferences from integration of pop-up satellite archival tagging studies.

Data sets from three laboratories conducting studies of movements and migrations of Atlantic swordfish (Xiphias gladius) using pop-up satellite archival tags were pooled, and processed using a common methodology. From 78 available deployments, 38 were selected for detailed examination based on deployment duration. The points of deployment ranged from southern Newfoundland to the Straits of Florida. The aggregate data comprise the most comprehensive information describing migrations of swordfish in the Atlantic. Challenges in using data from different tag manufacturers are discussed. The relative utility of geolocations obtained with light is compared with results derived from temperature information for this deep-diving species. The results show that fish tagged off North America remain in the western Atlantic throughout their deployments. This is inconsistent with the model of stock structure used in assessments conducted by the International Commission for the Conservation of Atlantic Tunas, which assumes that fish mix freely throughout the North Atlantic.

Age and Growth of Canadian East Coast Pollock: Comparison of Results from Otolith Examination and Mark–Recapture Studies

Abstract Pollock Pollachius virens were marked and released at relatively young ages in Canadian Maritime waters during 1979–1984. Substantial (n = 694) recaptures provided a unique opportunity for validation of age determinations made from examination of otoliths. Comparisons of mean lengths at age (age as determined from otolith examination) and the lengths at the assumed age (age at release, plus years at liberty) of recaptured fish indicated no significant differences. However, some (n = 211) of the recaptured tagged fish were also returned with the corresponding otoliths for each individual, and age determinations were completed. Thus, individual comparisons of otolith-derived ages and assumed ages were possible for those fish. Such comparisons revealed a tendency for the ages derived from otolith examination to exceed the assumed age by a year. Though these findings indicate a potential bias in age determinations during the 1980s, results from stock assessments indicate that strong and weak cohorts ...