Research Paper On Harp Seals

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Abstract

As the northern hemisphere continues to warm, declines in sea ice seriously impact species that rely on ice for reproduction and/or feeding. One such species, the Northwest Atlantic harp seal, gives birth on ice and feeds along the southern edge of the seasonal pack ice. Unfortunately, little is known about the impact of climate change on ice-dependent species in sub-Arctic areas, although they are areas of rapid ecosystem change. While climate change has been shown to affect harp seals directly through increased mortality of young, it may also impact indirectly through changes in prey and subsequent reproductive rates. Over the past four decades, harp seals have also undergone a large change in abundance, increasing from <1.5 million seals in the early 1970s to ∼7.4 million seals today and, since 1987, late-term abortions have been observed. To determine the importance of biological and environmental factors influencing reproduction, pregnancy and abortion rates of harp seals were estimated from samples collected between 1954 and 2014 off Newfoundland, Canada. Since the early 1980s, late-term pregnancy rates among mature females have declined while interannual variability increased, ranging from 0.2 to 0.86. Using a beta-regression model, we found that while the general decline in pregnancy is associated with increased population size, including the rate of late-term abortions captured much of the interannual variability. Changes in abortion rates were described by a model that incorporated capelin biomass and mid-winter ice cover (likely a proxy for ecosystem changes in overall prey abundance). Harp seals appear to respond to relatively small variations in environmental conditions when they are at high population levels. If the observed changes in climate continue, negative impacts on the Northwest Atlantic harp seal population will likely increase due to the predicted warming trend and associated reduction in ice cover.

Introduction

Population dynamics are influenced by changes in survival and reproductive rates that are the result of a complex interaction between intrinsic factors related to changes in the population (i.e. density-dependent) and extrinsic factors involving environmental variability (i.e. density-independent; e.g. Eberhardt, 1977 ; Gaillard et al ., 2000 ; de Little et al ., 2007 ). As species encounter changing environmental conditions as a result of climate change, understanding the influence of these different factors becomes critical if we wish to predict how a species will respond. Unfortunately, determining the relative importance of these different factors is difficult for most species as they require extensive, long-term, measurements of reproductive rates, population density or size, and a variety of environmental factors.

Environmental conditions throughout the world are changing rapidly, particularly in Arctic areas where over the past several decades, changes in temperature, ocean circulation, pH balance, ice cover, and sea level have been documented in response to global climate change (e.g. Walsh, 2008 ; IPCC, 2013 ). Perhaps the most dramatic changes have been in the extent of summer sea ice. As the planet continues to warm, these changes will continue, resulting in serious impacts on a number of species that inhabit ice covered areas for reproduction and/or feeding during all, or part of the year. A number of studies have examined the potential impact of climate change on marine mammals in the Arctic (e.g. Learmonth et al ., 2006 ; Laidre et al ., 2008 ; Kovacs et al ., 2011 ). However, with relatively few exceptions (e.g. Sundqvist et al ., 2012 ; Stenson and Hammill, 2014 ), few studies have examined impacts of climate change on sub-Arctic populations although the changes associated with climate change are likely to be most rapid along the ice edge that they inhabit ( Walsh, 2008 ). This is surprising as many of these ecosystems have been studied extensively due to their importance to commercial fisheries. As such, there are often data available to examine the impacts of changing environments in these areas in more detail than in polar regions.

The harp seal ( Pagophilus groenlandicus ) is the most abundant marine mammal species in the North Atlantic. They are an ice-dependent species, giving birth and nursing their pups on pack ice each spring. Their annual movements appear to follow the development and retreat of the ice pack, and they are often seen feeding along the ice edge throughout the year. The Northwest Atlantic (NWA) population ranges from the eastern Canadian Arctic and Baffin Bay in the north to the Gulf of St Lawrence in the south (Figure 1 ; Sergeant, 1991 ; Stenson and Sjare, 1997 ). Although water temperatures have varied historically, over the past four decades there has been a trend towards warmer water temperatures throughout their range. Associated with this warming trend there has been a decline in ice extent and coverage, particularly in their southern wintering and pupping area, with 4 of the past 7 years being among the lowest recorded ( Johnston et al ., 2005 , 2012 ; Friedlaender et al ., 2010 ; Bajzak et al ., 2011 ; Stenson and Hammill, 2014 ). Poor ice conditions (extent, coverage, and/or thickness) have been shown to impact harp seals directly; Stenson and Hammill, (2014) found that pup mortality was extremely high in years with reduced ice coverage or thickness.

Figure 1.

Seasonal distribution of NWA harp seals. Reproductive samples were obtained from October to February from southern Labrador and northern Newfoundland. This figure is available in black and white in print and in colour at ICES Journal of Marine Science online.

Figure 1.

Seasonal distribution of NWA harp seals. Reproductive samples were obtained from October to February from southern Labrador and northern Newfoundland. This figure is available in black and white in print and in colour at ICES Journal of Marine Science online.

NWA harp seals undertake an annual cycle of feeding, reproduction, and migrations (Figure 2 ). In late February and early March, females give birth to a single pup on the pack ice off southern Labrador and northeastern Newfoundland (referred to as “the Front”), or in the Gulf of St Lawrence (“the Gulf”). Approximately two-thirds of the pupping occurs at the Front ( Stenson et al. , 2014 ) and is highly synchronized with over 90% of the pupping occurring between 6th and 12th March. Pups are nursed for only 10–12 days ( Stewart and Lavigne, 1984 ) after which mating occurs. Seals 1 year of age and older undergo their annual moult between mid-April and late May ( Sergeant, 1991 ). Following a brief period of feeding, the seals migrate northward during late June or July to their summering areas in the Arctic waters of eastern Canada and western Greenland. In November and December, most harp seals migrate southward to overwinter off the coast of Newfoundland and in the Gulf of St Lawrence where they feed before pupping ( Stenson and Sjare, 1997 ).

Figure 2.

Annual cycle of pregnancy, movements, and weight gain in NWA harp seals. Harp seals undergo major weight gain during winter while in southern waters. Reproductive samples used in this study were collected from October to February. Winter prey biomass data are available from DFO bottom trawls carried out between October and January.

Figure 2.

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