Evaluating the potential impacts of grey seal predation and fishery bycatch/discards on cod productivity on the Western Scotian Shelf and in the Bay of Fundy

The recovery of many groundfish stocks throughout the Northwest Atlantic has been impeded by elevated natural (i.e., non-fishing) mortality (M) among older/larger individuals. The causes of elevated mortality are not well known, though predation by rapidly growing grey seal herds and unreported fishing are thought to be possible drivers of mortality for Atlantic Cod (Gadus morhua) on the Western Scotian Shelf and in the Bay of Fundy (known as "4X5Y cod") and in nearby ecosystems. We developed a statistical catch-at-age model for 4X5Y cod that accounted for both grey seal predation and estimated bycatch/discards to evaluate the degree to which either of these factors may influence cod mortality. The model was fit over a range of predation and discarding scenarios to account for uncertainties and a lack of data for these processes. We found that most cod M remained unaccounted for unless cod comprised a large proportion (>0.45) of the grey seal diet by weight. If the reported bycatch estimates are taken as accurate, then the magnitude of cod discards from non-directed fisheries was minor, though these estimates are highly uncertain.Comment: 27 pages, 4 tables, 10 figures, 2 appendice

Organic matter mineralization in lake sediments : a within and among lake study

Organic matter mineralization by sediment bacteria was measured by the accumulation of DIC + CH$sb4$ in the water overlying intact cores taken from littoral and profundal sediments of 9 lakes. The variability in areal carbon mineralization was much greater within lakes than among lakes, with the rate of organic matter mineralization in littoral sediments, on average, 3-fold higher than in the deeper sediments.Sixty percent of the variation in summer carbon mineralization rates is explained by site depth, a surrogate variable which incorporates the effect of temperature and may also be reflecting organic matter quality and/or supply. Lake-specific variables become useful predictors of carbon mineralization only after the site depth is considered.A comparison of the mineralization in sediments overlain by epilimnetic water to the whole lake sediment mineralization demonstrates the overwhelming importance of the littoral sediments in organic matter mineralization, with more than half (54-100%) of the mineralization in the sediments occurring in the littoral zone. However, the littoral sediments account for less than 20% of the gross respiration in the epilimnion. (Abstract shortened by UMI.

Ontogeny of movement patterns in naïve grey seal pups inhabiting a complex continental shelf ecosystem.

Most vertebrate offspring must transition from the relative security of parental care (nutrition and protection) to independent foraging. Offspring face many challenges during this critical period, particularly in species where parental care ends at weaning, such as the grey seal (Halichoerus grypus). We studied the development of movement behaviour in naïve grey seal pups from their first trips to sea to about five months of age. Twenty-five (12 males and 13 females) newly-weaned pups were fitted with satellite-linked GPS tags on Sable Island, Nova Scotia, Canada in January 2016. The influence of fixed effects (pup size, sex, week) and the random effect of pup identity on trip characteristics were examined. Movement behaviour was analyzed using a move persistence mixed-effects model. Habitat use was highly variable among individuals and covered much of the geographic distribution of the population. Unlike older juveniles, subadults, and adults in this population, most naïve pups used multiple haulout sites to begin and end trips. There was little evidence of area-restricted search behaviour during trips, suggesting that naïve pups were using an opportunistic foraging tactic that may result in more variable foraging success than that of older, experienced animals. Naïve pups made longer trips with longer haulout durations between them than observed for older greys seals. Males and females differed in some trip characteristics, but sex effects were small over the first few months of life. Offspring size at weaning was not a useful predictor of trip characteristics. Move persistence of grey seal pups was initially high and then decreased over time as individuals gained experience. Both intrinsic and extrinsic factors were influential on the movements of grey seal pups. Greater body length at weaning, longer duration spent on shore after weaning, shallower water column depth, and farther distance from shore were all associated with lower move persistence. Female grey seal pups had lower move persistence than males. Overall, the movements of naïve grey seal pups during the first few months of life were characterized by extensive exploration, but move persistence decreased over time suggesting they may be using an exploration-refinement foraging tactic

Auto-correlation function (ACF) plots of one-step-ahead residuals for longitude and latitude from the best-supported move persistence mixed-effects model.

Auto-correlation function (ACF) plots of one-step-ahead residuals for longitude and latitude from the best-supported move persistence mixed-effects model.</p

Model selection for the move persistence mixed effects model fitted to 22 naïve grey seal pup tracks interpolated at a six-hour time step and estimate move persistence (γ_t).

Fixed effects: week = week of tracking, body mass and length at weaning, postwean = time between weaning and first trip to sea, distshore = distance nearest 0 m isobath, and depth = ln-transformed water column depth. Pup identity is included as a random intercept. The number of degrees of freedom (df), log-Likelihood (LL), second-order Akaike Information Criterion (AICc), difference in AICc from that of the best-supported model (ΔAICc), and Akaike weight (wi) are presented. The best-supported model is indicated in bold. Coefficient estimates, standard errors, and p-values of predictor variables are shown for the best-supported move persistence mixed-effects model. Coefficients with significant p-values are indicated in bold. The quantile range (5% and 95%) of parameter estimates from leave-one-out cross validation of the most parsimonious model are presented with the estimated trend (Est Trend) which represents the percentage of cross validation models where the estimated coefficients fall within the 95% confidence intervals of the parameter estimates from the model including all individuals.</p

S3 Fig -

Plots of the one-step-ahead residuals for longitude and latitude from the best-supported move persistence mixed-effects model and the covariates week (top), distance to shore (middle), and ln-transformed depth (bottom). There are no obvious patterns between the residuals and covariates other than some evidence for unequal variance, which suggests that the linear assumption is adequate. (PDF)</p

Fig 4 -

Fastloc GPS tracks from 22 naïve grey seal pups instrumented with SPLASH 10-AF satellite-linked transmitters (www.wildlifecomputers.com) on Sable Island during the 2016 breeding season showing locations interpolated at a six-hour time step coloured as a gradient for (a) move persistence (γt) estimated using the best-supported move persistence mixed-effects model and (b) weeks at sea.</p

Fig 1 -

Bathymetry of the Scotian Shelf and surrounding regions with the 100 m and 200 m isobaths (black lines), Sable Island (red circle), and topographical features mentioned in text: (1) Gulf of Maine (2) LaHave Basin, (3) Emerald Basin, (4) Emerald Bank, (5) Sable Bank, (6) Middle Bank, (7) Misaine Bank, (8) Banquereau Bank, (9) the Laurentian Channel, (10) Gulf of St. Lawrence, (11) Western Shelf, and (12) Grand Banks. Bathymetric data acquired from the NOAA ETOPO1 database [59].</p

Move persistence estimates (γ_t) from the best-supported move persistence mixed-effects model fitted to Fastloc GPS location data from 22 naïve grey seal pups interpolated at a six-hour time step.

Plots display individual (coloured lines) random effects and group (blue line) fixed effects responses for continuous predictors and include weaning body mass (kg) and length (cm), the post-weaning duration (d), week number, the distance to nearest shore (km), and ln-transformed water column depth (m); the effect of sex is not shown.</p