Browsing by Subject "Atlantic Cod"

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Open Access
Characterization of Scale in Commercial Fisheries Data
(2004) Sanderson, Melissa A
As management of commercial fisheries becomes more spatially oriented, collection of commercial fisheries data must adapt to accurately reflect location. An important component of accurate spatial data is scale. In an attempt to characterize patterns of scale in fisheries data, I tested the National Marine Fisheries Service Northeast bottom trawl survey data for spatial dependency using semivariogram analysis. Specifically, and more importantly for management, I wanted to determine if the distance between sample locations is a good predictor variable for how much fish will be caught. Focusing on 1996-2002 catch data for Atlantic cod and witch flounder, I found that for current data collection techniques, the variance of catch weight is spatially independent from distance between observations. Thus, the scale and spatial pattern of the data can not be characterized based on distance for the range of space and time analyzed. This finding does not rule out the possibility that spatial dependence may be observed in these fisheries if we were to examine data sets with finer spatial distances and finer time intervals. Because ocean processes vary significantly across time, the effect of aggregating the spatial data across time may have acted to conceal some of the potential trends in the data set. Determining the spatial patterns in the data is part of a sequential approach to understanding ecological processes. Alternative hypotheses that may possibly explain the spatial pattern of the data need to be tested and include spatial patterns being dependent upon bottom habitat complexity, water temperature, and/or prey availability. The goal is to find a variable that explains fish biomass patterns, allowing managers and scientists to begin to understand what proxy data they really need to collect and map, and at what scale, in order to predict patterns of fishes for effective and sustainable fisheries management.
Open Access
Examining Space Use of Foraging Gray Seals (Halichoerus grypus) off the New England Coast
(2021-04-30) Smith, Beatrice C.
The Gray seal (Halichoerus grypus) is a highly migratory large phocid species occurring in the waters of the Northeast Atlantic Ocean. After severe depletion during the 19th and 20th century, this species has recolonized southerly US habitats around the Gulf of Maine (GOM) and Cape Cod, resulting in increasing conflict with commercial fisheries that overlap with seal foraging grounds. With uncertainty surrounding gray seal population dynamics as well as methods for the management of this ecologically important protected predator, there is an incentive to expand knowledge of gray seal feeding ecology, diet and foraging behavior to better understand interactions between this species and economically profitable fishing sectors. This study investigates gray seal foraging behavior off the coast of Cape Cod and throughout the GOM. Examining data from 4 tagged individuals, this analysis implements a potential path volume (PPV) methodology (Demšar and Long, 2019), to estimate space use in order to assess the effect of sex and season on seal foraging volume. Additionally, a generalized additive model (GAM) was used to predict species distribution and habitat suitability for a commercially important gray seal prey item, Atlantic cod, to better understand potential spatial overlap between commercial fisheries and foraging seals. Results from PPV analysis shows highly variable space use by foraging seals, with greater overall volumetric range during the Spring/Summer months. Spatial analysis of overlap between foraging seals and predicted higher quality cod habitat was shown to vary by season. The methodologies used in this study to assess overlap between commercially valuable fish species and foraging gray seals have the potential to act as a valuable tool for managers and fishers moving forward. Additionally, seal PPV is a useful metric to consider when evaluating trends in foraging ecology of this species. Finally, the combination of the two methodologies implemented in this paper might help shed light on recovery of vulnerable cod stocks.