Search

Atlantic cod Gadus morhua populations in the northeast USA have failed to recover since major declines in the 1970s and 1990s. To rebuild these stocks, managers need reliable information on spawning dynamics in order to design and implement control measures; discovering cost-effective and non-invasive monitoring techniques is also favorable. Atlantic cod form dense, site-fidelic spawning aggregations during which they vocalize, permitting acoustic detection of their presence at such times. The objective of this study was to detect spawning activity of Atlantic cod using multiple fixed-station passive acoustic recorders to sample across Massachusetts Bay during the winter spawning period. A generalized linear modeling approach was used to investigate spatio-temporal trends of cod vocalizing over 10 consecutive winter spawning seasons (2007-2016), the longest such timeline of any passive acoustic monitoring of a fish species. The vocal activity of Atlantic cod was associated with diel, lunar, and seasonal cycles, with a higher probability of occurrence at night, during the full moon, and near the end of November. Following 2009 and 2010, there was a general decline in acoustic activity. Furthermore, the northwest corner of Stellwagen Bank was identified as an important spawning location. This project demonstrated the utility of passive acoustic monitoring in determining the presence of an acoustically active fish species, and provides valuable data for informing the management of this commercially, culturally, and ecologically important species.

Atlantic halibut are a “Species of Concern” in US waters and little is known about their movements and stock structure. Recent stock assessments drew attention to the paucity of information for assessing and managing this stock. To investigate movement patterns and stock structure, halibut were tagged off Massachusetts and Maine within US waters of the Gulf of Maine region using pop-up satellite archival tags and data storage tags. A hidden Markov model (HMM) geolocation method previously developed for other groundfish species was adapted to estimate the movement tracks of the tagged halibut (n = 25) based on the tag-recorded depth and temperature. Total distance travelled based on geolocation ranged from 36 to 1701 km, whereas straight line distance between tagging and end locations ranged from 0.4 to 440.7 km. Estimated movement rates varied between 2.7 and 10 km day−1. Two tagged halibut made long-distance movements to putative spawning habitat in the Northeast Channel off Georges Bank. Thirteen (13) out of 25 geolocated individuals were estimated to have reached Canadian waters. Geolocation results revealed home range, return movement, and seasonal migration movement patterns exhibited by the tagged halibut. The HMM geolocation method could be a useful tool in providing information on halibut movements that can inform stock assessment and management decisions.

Models developed to geolocate individual fish from data recorded by electronic tags often require major modification to be applied to new regions, species, or tag types due to variability in oceanographic conditions, fish behavior, and data resolution. We developed a model for geolocating Atlantic cod (Gadus morhua) off New England that builds upon an existing hidden Markov model (HMM) framework and addresses region- and species-specific challenges. The HMM framework contains a likelihood model that compares tag-recorded environmental data (depth, temperature, tidal characteristics) with those derived from an oceanographic model and a behavior model that constrains the horizontal movement of the fish. Validation experiments were performed on stationary tags, double-electronic-tagged fish (archival and acoustic tags), and simulated tracks. Known data, including fish locations and activity metrics, showed good agreement with those estimated by the modified approach and improvements in performance of the modified method over the original. The modified geolocation approach will be applicable to additional species and regions to obtain valuable movement information that is not typically available for demersal fishes.

The spawning dynamics of Atlantic cod (Gadus morhua) on Georges Bank and Nantucket Shoals are not well understood. To address this uncertainty, we combined Fishermen’s Ecological Knowledge (FEK) with traditional scientific data to develop a more holistic understanding of cod spawning on Georges Bank. Data from historical reports, trawl surveys, fisheries observers, and ichthyoplankton surveys were used to describe the spatial and temporal distribution of cod spawning activity. We also collected FEK regarding cod spawning dynamics through semi-structured interviews (n = 40). The fishermen had detailed knowledge of the spatial and temporal distribution of cod spawning, and identified persistent fine-scale (i.e.

Movement patterns of marine fishes can have considerable impacts on their population dynamics. A thorough understanding of fish movements is therefore required for informing stock identification, stock assessment, and fishery management. This study investigated the seasonal movements and connectivity of a spring-spawning component of Atlantic cod (Gadus morhua) in the western Gulf of Maine. From 2010 through 2013, spawning cod were sampled within an inshore spawning closure and tagged with conventional tags (n = 2368), acoustic transmitters (n = 106), and archival data storage tags (n = 266). Acoustic receivers were deployed on three inshore spawning sites to test for connectivity among sites. Data from archival tags were used to describe seasonal habitat occupancy and movement patterns via geolocation to statistical areas. Tagging data indicated that cod were primarily residential in the western Gulf of Maine, moving inshore to spawn during the spring (April–July), followed by an offshore migration to their feeding grounds for summer and fall. Cod generally inhabited waters from 45 to 175 m, with the deep offshore basins (>150 m) serving as overwintering habitat. Occupied water temperatures ranged from 4.0 to 13.3 °C, with the coldest temperatures experienced from March through July and the warmest temperatures experienced from September through January. Results provided evidence of spawning site fidelity and connectivity among spawning sites, with some fish visiting multiple spawning sites within or between years. The movements observed during and after the spring-spawning season serve as important mechanisms influencing metapopulation dynamics in the Gulf of Maine region, including both fine- and broad-scale population structure. The improved understanding of cod movement patterns will assist fishery managers in developing management plans, including spawning protection measures, and help to address remaining uncertainties with respect to cod population structure in the Gulf of Maine and other regions.

Atlantic cod (Gadus morhua) populations in the Gulf of Maine (GoM) are at a fraction of their historical abundance, creating economic hardships for fishermen and putting at risk the genetic diversity of the remaining populations. An understanding of the biocomplexity among GoM populations will allow for adaptive genetic diversity to be conserved to maximize the evolutionary potential and resilience of the fishery in a rapidly changing environment. We used restriction-site-associated DNA sequencing (RADseq) to characterize the population structure and adaptive genetic diversity of five spawning aggregations from the western GoM and Georges Bank. We also analyzed cod caught in the eastern GoM, an under-sampled area where spawning aggregations have been extirpated. Using 3,128 single nucleotide polymorphisms (SNPs), we confirmed the existence of three genetically separable spawning groups: (1) winter spawning cod from the western GoM, (2) spring spawning cod, also from the western GoM, and (3) Georges Bank cod. Non-spawning cod from the eastern GoM could not be decisively linked to either of the three spawning groups and may represent a unique component of the resource, a mixed sample, or cod from other unsampled source populations. The genetic differentiation among the three major spawning groups was primarily driven by loci putatively under selection, particularly loci in regions known to contain genomic inversions on linkage groups (LG) 7 and 12. These LGs have been found to be linked to thermal regime in cod across the Atlantic, and so it is possible that variation in timing of spawning in western GoM cod has resulted in temperature-driven adaptive divergence. This complex population structure and adaptive genetic differentiation could be crucial to ensuring the long-term productivity and resilience of the cod fishery, and so it should be considered in future management plans.

In recent years, the recreational contribution to the total catch of Atlantic cod (Gadus morhua) in the Gulf of Maine (GOM) has increased with recreational discards outnumbering recreational landings by 2:1. However, the discard mortality (DM) rate of cod released in the recreational fishery remains poorly understood, thus contributing to the uncertainty in stock assessments and fishery management plans. The current study examined the capture-related factors most detrimental to cod DM in the GOM recreational rod-and-reel fishery. Atlantic cod (n = 640; 26.0–72.0 cm) were angled from June–October 2013 on southern Jeffreys Ledge in the western GOM using fishing gear representative of the local recreational fishery. A subset (n = 136) was also tagged with pressure-sensing acoustic transmitters before being released into an acoustic receiver array (n = 31) deployed to monitor survival up to 94 days. To properly model DM up to the fishery-wide level, all cod were visually assessed for capture-related injuries according to a four-level injury score index. Mean tackle-specific DM rates of 15.4 and 21.2% were estimated for bait- and jig-captured cod, respectively, with an overall 16.5% mean DM rate for the 2013 GOM recreational cod fishery. Twenty-nine cod tagged with acoustic transmitters were identified as dead, where the majority (∼90%) died within 16 h post-capture. Upon evaluation with a specifically adapted parametric survival analysis, greater incidence of mortality was attributed to the capture and handling process (rather than release) for moderately and severely injured cod. Based on the capture-related factors associated with the highest injury rates, we recommend minimizing fight and handling times, avoiding areas with small cod, educating inexperienced anglers, and favouring bait over jigs to mitigate mortality. Results will continue to inform the development of fishery management plans and enhance survival through dissemination of “best practice” techniques to fishery stakeholders.

Conservation concerns and new management policies such as the implementation of ecosystem-based approaches to fisheries management are motivating an increasing need for estimates of mortality associated with commercial fishery discards and released fish from recreational fisheries. Traditional containment studies and emerging techniques using electronic tags on fish released to the wild are producing longitudinal mortality-time data from which discard or release mortalities can be estimated, but where there may also be a need to account analytically for other sources of mortality. In this study, we present theoretical and empirical arguments for a parametric mixture-distribution model for discard mortality data. We show, analytically and using case studies for Atlantic cod (Gadus morhua), American plaice (Hippoglossoides platessoides), and winter skate (Leucoraja ocellata), how this model can easily be generalized to incorporate different characteristics of discard mortality data such as distinct capture, post-release and natural mortalities, and delayed mortality onset. In simulations over a range of conditions, the model provided reliable parameter estimates for cases involving both discard and natural mortality. These results support this modelling approach, indicating that it is well suited for data from studies in which fish are released to their natural environment. The model was found to be less reliable in simulations when there was a delay in discard mortality onset, though such an effect appears only in a minority of existing discard mortality studies. Overall, the model provides a flexible framework in which to analyse discard mortality data and to produce reliable scientific advice on discard mortality rates and possibilities for mitigation.

Rebuilding the Gulf of Maine stock of Atlantic cod (Gadus morhua) has been much slower than expected. An important source of scientific uncertainty contributing to the difficulties in managing rebuilding has been the lack of understanding of cod population structure. Previous research indicates that the stock functions as a metapopulation that is made up of multiple subpopulations and many finer-scale spawning components. This study investigated fine-scale, multiyear spawning site fidelity by a spring-spawning component of Atlantic cod in the western Gulf of Maine. Movements of acoustically tagged cod (n = 63) with respect to a known spawning site were tracked using passive acoustic telemetry. A large proportion (38–67%) of tagged cod exhibited spawning site fidelity between 2010 and 2012. After adjusting for fishing mortality, natural mortality, and skipped spawning, the estimated rate of spawning site fidelity ranged between 47 and 95% in 2011. Multiyear spawning site fidelity was also observed, with individuals being tracked for up to four consecutive spawning seasons. Spawning site fidelity serves as one of the multiple mechanisms that contribute to the formation and maintenance of the observed metapopulation structure. Spawning site fidelity also reduces the reproductive connectivity among spawning sites, thus delaying both recolonization of abandoned spawning sites and stock rebuilding. Future stock assessment models and fishery management plans that incorporate the metapopulation structure of cod in the Gulf of Maine are expected to be more effective at preventing continued declines in spawning diversity and promoting rebuilding.

Understanding the influence of spawning behaviour on the fine-scale distribution of Atlantic cod is essential to the design of effective conservation measures. Laboratory studies suggest that spawning activity occurs primarily at night, yet no field studies have evaluated the influence of diel period on the behaviour of individual wild spawning cod. Using an acoustic telemetry positioning system, the fine-scale movements of spawning cod were observed in situ as they returned to the same spawning location over consecutive seasons. The resulting data identify clear gender-based diel patterns in space use and aggregation behaviour among cod on a spawning ground. During the day, females remained aggregated in one small location that varied little within and between years. Males also aggregated during the day, but occupied a much larger adjacent area. At night, individual males sought out separate small territories while females generally remained near their daytime aggregation site, making periodic excursions into the surrounding area. These patterns were surprisingly stable over the 2 years of observation, indicating little interannual variability in spawning behaviour. This study provides an unprecedented examination of the natural spawning behaviour of Atlantic cod, and makes connections between earlier laboratory studies and field observations.

Mismatches between biological population structure and management unit boundaries often violate the unit-stock assumption, which can reduce the accuracy and relevance of stock assessment results and lead to ineffective fishery management. Since 1972, Atlantic cod (Gadus morhua) have been managed in US waters as two units: the Gulf of Maine and the Georges Bank stocks, both of which have experienced recent difficulties in rebuilding. An interdisciplinary review of available biological information was conducted to investigate cod population structure in US waters and to evaluate the biological appropriateness of the current two-stock model. Our review demonstrates that spawning components in the Great South Channel, Nantucket Shoals, southern New England, and Middle Atlantic are more connected with spawning components in the Gulf of Maine than on eastern Georges Bank, with which they are currently managed. Therefore, a modification of current stock boundaries is recommended to provide a more accurate representation of biological population structure. Proposed alternatives divide inshore and offshore spawning components into separate management units, thereby separating the current Georges Bank stock longitudinally. Continued research, including stock composition analysis, is required to evaluate uncertainties, delineate biological stocks, and develop sustainable management practices that account for intrastock diversity (e.g. winter and spring-spawning components that overlap spatially).

Many stocks of Atlantic Cod Gadus morhua have been depleted due to interactions between overfishing and environmental variation. Stock declines were often accompanied by reductions in spawning diversity and collapses of population structure. Studying Atlantic Cod spawning activity can provide insights into the mechanisms involved in rebuilding. This paper reviews the existing literature on Atlantic Cod spawning dynamics, including habitat, timing, behavior, gamete production, larval survivorship, and fishery impacts. The associated implications for fisheries management are discussed, and critical information gaps are identified for inclusion in future research. Of primary importance for consideration by fisheries managers are the spatial and temporal extent of spawning, the behavior of spawners, and the reproductive contributions of older age-classes. The application of spawning closures as part of a multidisciplinary approach to fisheries management is advocated to prevent the disruption of spawn...

Atlantic cod form spawning aggregations in locations and seasons that are persistent from year to year and individual fish have been shown to exhibit spawning site fidelity and home to specific spawning grounds each season. In the Gulf of Maine, cod are known to have historically occupied a mosaic of spawning grounds but many of these spawning components have been extirpated, primarily through overfishing, with a near complete loss of spawning along mid-coast and eastern Maine. The remaining spawning aggregations in the western Gulf of Maine are particularly vulnerable to over-exploitation owing to their proximity to shore, the predictability of their timing, the fine-scales upon which they operate, and the high density of fish within each aggregation. Broad scale management actions that are currently being discussed may allow an increased harvest from these spawning aggregations. In this paper we describe the creation of three small-scale area closures that serve to eliminate the exploitation and disturbance of discrete spawning aggregations of Atlantic cod and prevent the potential extirpation of these spawning components. Each closure was unique in the circumstances that surrounded their creation, including differences in the amount of prior protection from commercial and recreational exploitation, the timing and duration of the closure, the size of the closure area, the management body that had authority to enact the closure, the amount of monitoring that has occurred, and the amount of spatial or temporal modifications that have occurred since enactment. We believe the case for spawning closures for Atlantic cod has already been made by several authors and the purpose of this paper is not to present new science, but rather to show the path that was followed to create these spawning closures within the complicated array of fisheries management.

Hernandez, K. M., Risch, D., Cholewiak, D. M., Dean, M. J., Hatch, L. T., Hoffman, W. S., Rice, A. N., Zemeckis, D., and Van Parijs, S. M. 2013. Acoustic monitoring of Atlantic cod (Gadus morhua) in Massachusetts Bay: implications for management and conservation. – ICES Journal of Marine Science, 70: 628–635. Atlantic cod (Gadus morhua) stocks in northeastern US waters are depleted and stock recovery has been slow; research into the spawning behaviour of this species can help inform conservation and management measures. Male cod produce low-frequency grunts during courtship and spawning. Passive acoustic monitoring (PAM) offers a different perspective from which to investigate the occurrence, spatial extent and duration of spawning cod aggregations. A marine autonomous recording unit was deployed in the “Spring Cod Conservation Zone” (SCCZ) located in Massachusetts Bay, western Atlantic, to record cod grunts from April–June 2011. Cod grunts were present on 98.67% of the recording days (n = 75 days). They occurred across all 24 h, although significantly more grunts were found during the day than night-time (p = 0.0065). Grunt durations ranged from 57–360 ms, and the fundamental frequency and second harmonic had mean peak frequencies of 49.7 ± 5.6 and 102.9 Hz ± 10.9 sd, respectively. Cod grunt rates were low compared with those reported for other spawning fish, and may be indicative of diel movement patterns. Next steps will focus on expanding PAM coverage within the SCCZ, alongside prospecting for unknown spawning grounds within existing archival data.