Population Dynamics of Four Northern Seals: Population Estimates and the Influence of Internal and External Factors on Population Flux

Defense Date

8-2001

Document Type

Capstone

Degree Name

M.S. Marine Biology

First Advisor

Keith Ronald

Second Advisor

Curtis Burney

Abstract

Accurate estimates of seal abundance and population dynamics are necessary for management and conservation initiatives. Population growth rate and size are the result of internal factors such as age-specific fertility and pregnancy rates. These internal factors can in turn be influenced by external factors such as pollution, predation, food availability and density dependence, and hunting. All of these factors impact fecundity and mortality and lead to population flux. Seals feeding on fish contaminated with high levels of organochlorines can develop reproductive failure, resulting in lower pregnancy rates. These pollutants can also induce immunosuppression, leading to increased susceptibility of seals to disease. Predation by sharks, in some areas, can cause high mortality in newborns and reproductive females, resulting in lowered reproductive rates. Hunting can cause drastic declines in seal populations. The influence of all of the above factors on populations of harp seals (Pagophilus groenlandica), hooded seals (Cystophora cristata), grey seals (Halichoerus grypus), and harbor seals (Phoca vitulina) over the past twenty years was reviewed. There are believed to be several distinct stocks among each of these species. At this time, all of the stocks are either stable or increasing. Several stocks are recovering steadily from heavy exploitation. Hooded seal and harbor seal populations are increasing slowly, grey seal populations are increasing rapidly, and harp seal populations have leveled off at very high numbers. The increasing seal populations have raised concerns of their impact on commercial fish stocks. However, there is no conclusive evidence that seals are responsible for diminishing fish stocks. The populations of these four seal species are doing well, but accurate data on production, reproductive parameters and catch levels (including by-catch and struck and lost) are still needed to properly manage these populations in the future.

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