Modeling Recruitment of Birth Cohorts to the Breeding Population: A Hidden Markov Model Approach

Long-term capture-recapture studies provide an opportunity to investigate the population dynamics of long-lived species through individual maturation and adulthood and/or time. We consider capture-recapture data collected on cohorts of female gray seals (Halichoerus grypus) born during the 1990s and later observed breeding on the Isle of May, Firth of Forth, Scotland. Female gray seals can live for 30+ years but display individual variability in their maturation rates and so recruit into the breeding population across a range of ages. Understanding the partially hidden process by which individuals transition from immature to breeding members, and in particular the identification of any changes to this process through time, are important for understanding the factors affecting the population dynamics of this species. Age-structured capture-recapture models can explicitly relate recruitment, and other demographic parameters of interest, to the age of individuals and/or time. To account for the monitoring of the seals from several birth cohorts we consider an age-structured model that incorporates a specific cohort-structure. Within this model we focus on the estimation of the distribution of the age of recruitment to the breeding population at this colony. Understanding this recruitment process, and identifying any changes or trends in this process, will offer insight into individual year effects and give a more realistic recruitment profile for the current UK gray seal population model. The use of the hidden Markov model provides an intuitive framework following the evolution of the true underlying states of the individuals. The model breaks down the different processes of the system: recruitment into the breeding population; survival; and the associated observation process. This model specification results in an explicit and compact expression for the model with associated efficiency in model fitting. Further, this framework naturally leads to extensions to more complex models, for example the separation of first-time from return breeders, through relatively simple changes to the mathematical structure of the model.

Flatfish may be underestimated in the diet of gray seals (Halichoerus grypus)

The United States east coast population of the gray seal (Halichoerus grypus (Fabricius, 1791)) was once hunted to near extirpation, but the population has since rebounded due to protection by the Marine Mammal Protection Act of 1972. Although this population growth is seen as a success by conservationists, others are concerned about the economic and ecological impact of gray seals on New England fisheries. The study objective was to quantify flatfish presence in the diet of gray seals using an analysis of prey DNA in seal scat (molecular scatology). This may reduce a potential bias in other diet analysis methods since flatfish may not be swallowed whole, and therefore, their identifying otoliths may not be present. Scats were collected from Muskeget and Monomoy islands off Cape Cod, Massachusetts, USA, in 2016, 2017, and 2018 and analyzed for flatfish presence using species-specific polymerase chain reaction (PCR) assays and a hard parts analysis using otoliths. Frequency of occurrence for flatfish species from DNA was higher than estimated in previous literature on gray seal diet and a concurrent otolith analysis of the same samples, suggesting that previous analyses may have potentially underestimated the importance of flatfish in the diet.

Daily and seasonal movements of Cape Cod gray seals vary with predation risk

White sharks Carcharodon carcharias and gray seals Halichoerus grypus are re-establishing their ecological roles within the Northwestern Atlantic Ocean, presenting an opportunity to understand gray seal movement and at-sea behavior under predation risk. As with other shark-seal hotspots, movements to and from terrestrial haul outs can be risky for gray seals, thereby eliciting antipredator strategies. We investigated the movement and coastal behavior of gray seals on Cape Cod (USA) in relation to seasonal and diel changes in white shark activity. Analyzing 412 trips to sea by 8 seals and more than 25000 acoustic detections from 23 individual white sharks, we observed seasonally homogeneous movements in seal behavior during months with greater shark presence. During riskier months, seal behavior manifested in near-exclusive nocturnal foraging, reduced offshore ranging, and limited at-sea activity. On these nocturnal trips to sea, seals returning to haul outs tended to avoid daybreak and traversed during diel minima in shark activity. However, seals tended to depart haul outs at dusk when shark presence was maximal. As conservation efforts succeed in rebuilding depleted populations of coastal predators, studying re-emerging predator-prey interactions can enhance our understanding about the drivers of movement and behavior.

Research on the ability of grey seals to differentiate light sources with different wavelengths.

The article investigated the ability of gray seals to develop a differentiating conditioned reflex to light sources with different wavelengths corresponding to red, yellow, green, cyan and blue colors. According to the obtained data, light sources with different wavelengths can serve as an irritant for the gray seal, which produces a conditional differentiation reflex. The development of a differentiating conditioned reflex to the light source occurs in the studied gray seals much faster than on color painted objects. The results of the experiment showed the presence of protanopia in the studied gray seals.

Rates of increase in gray seal (Halichoerus grypus atlantica) pupping at recolonized sites in the United States, 1988–2019

Gray seals were historically distributed along the northeastern coast of the United States, but bounties and lack of protection reduced numbers and they were rarely observed for most of the 20th century. Once protections were enacted, the population started to rebound. Here, we describe the recolonization and recovery of gray seals in the United States, focusing on the re-establishment of pupping sites. We fit individual generalized linear models to various time series (1988–2019) to estimate rates of increase in observed pup counts at four of the more data-rich sites. Annual rate of increase at individual sites ranged from −0.2% (95% CI: −2.3–1.9%) to 26.3% (95% CI: 21.6–31.4%). The increase in sites and number of pups born in the United States is driven by population growth and immigration from Canadian colonies and is part of a larger recovery of the Northwest Atlantic population. Wildlife protection, a healthy source population, habitat availability, and species traits that allow for dispersal and high productivity were all important factors in this recovery.