North Atlantic right whale (Eubalaena glacialis) recovery strategy: chapter 9
1.5 Biological Limiting Factors
1.5.1 Life history strategy
Right whales are typical of species that exhibit long life spans, mature relatively late and produce fewer, larger young. These characteristics result in relatively long generation times. Offspring are typically relatively large and develop slowly, requiring a fair degree of parental investment. Such a life history strategy allows the population to endure periods of low reproductive output. However, long generation times and low annual reproductive rates may leave the species susceptible to increases in mortality (e.g. human induced). Over longer periods these species are prone to extinction if mortality remains high. Population recovery can require long periods (on the order of decades) due to the low reproductive rates.
1.5.2 Low genetic diversity
Reproductive success in North Atlantic right whale may be impaired by low genetic diversity in the population. The ongoing genetic analyses of the right whale have shown that this species has among the lowest levels of genetic diversity identified in a large mammal at all markers tested to date, including: minisatellite markers (Schaeff et al. 1997); mitochondrial DNA sequences (Malik et al. 2000); and microsatellite markers (Waldick et al. 2002). These findings have led to the hypothesis that the low level of genetic variability may be at least partially responsible for the reduced reproductive performance observed in this species. Therefore, because this species has such low levels of genetic variability there is an increased probability that mating pairs will have similar genetic profiles, which could subsequently result in a high rate of foetal loss and a reduced reproductive performance. Although conservation actions could not likely be implemented to mitigate the negative impacts of genetic characteristics on reproduction, this information will provide an estimate of the degree to which reproductive success and species recovery are limited by intrinsic factors.
Analyses of the genetic characteristics of the extant population did not detect any evidence of a genetic bottleneck (Waldick et al. 2002). These tests are only sensitive to recent events and suggest that a genetic bottleneck has not occurred since approximately the 1800s. Additionally, genetic analyses of historic specimens from the 1500s suggest that the impact of Basque whaling on the North Atlantic right whale was much less than has previously been assumed, and that small population size may be a long-term characteristic of this species (Rastogi et al. 2004, Frasier et al. 2007). In light of these recent data, suggesting that this species had a small population size prior to whaling, the idea of nutritional limitations becomes more feasible.
One hypothesis is that the last ice age reduced this population to a relatively small size, and additionally altered the environment in such a way that the western North Atlantic could only support a small population. Under this scenario, nutritional factors, as well as genetic factors associated with long-term small population sizes, would be expected to influence present-day population trends. Although the hypothesis of nutritional limitations (i.e. reduced availability of right whales’ copepod prey) remains to be tested, there is evidence that genetic characteristics are influencing reproductive success as would be expected under this scenario (Frasier et al. 2007).
1.5.3 Low reproductive rates
The reduced reproductive performance of the right whale is recognized as one of the primary factors limiting the species’ recovery. Both short-term and long-term factors may affect reproductive rates (Kraus et al. 2007), and may have different implications for long-term recovery.
In a recent analysis of calving intervals between 1982 and 2006, Kraus et al. (2007) observed significant variation in the amount of time between births by individual females. The short-term effects of reduced reproduction include an increase in the average inter-birth interval from approximately 3.5 years in the 1980s to approximately 6 years in the 1990s (Kraus et al. 2001), although since 2001 this average appears to be returning to approximately 3 years for some of the adult females (Kraus et al. 2007).
The long-term reproductive performance of this species is lower than expected, as is demonstrated in the lower number of calves born per year than would be expected given the population parameters. This productivity deficit can be illustrated with a simple calculation. In 2005 the US National Marine Fisheries Service (NMFS) estimated population abundance to range from 300 to 350 individuals (NMFS 2005). Sex ratio in this population is approximately 50:50 (Brown et al. 1994), and it is estimated that 60% of the females are adults (Hamilton et al. 1998). Application of these ratios to the population abundance results in an estimate of approximately 90-105 adult females in any given year. Therefore, since females are capable of giving birth once every 3 years (Knowlton et al. 1994, Kraus et al. 2001), it is expected that approximately 30-35 calves should be born per year, instead of the average of 11 in the 1980s and 1990s (Kraus et al. 2001) and 23 between 2001 and 2005 (Kraus et al. 2007) It appears that a high percentage of adult females have either never given birth or have had only one offspring.
1.6 Economic, Cultural and Ecological Significance
The right whale is an important resource for several coastal communities in Atlantic Canada (see whale watching below). The right whale is a large, rare species that is becoming better known amongst the general public. There has been some media coverage about conservation issues and actions in Canada and the United States. The species almost certainly has substantial non-consumptive value to Canadian society because members of the public want to preserve the species for future generations (legacy value) or just derive value from knowing the species exists even though they will never personally see or ‘use’ the species (existence value).
1.6.1 Whale watching
Whale watching is an important part of the growing ecotourism sector along the shores of New Brunswick and Nova Scotia, particularly in the West Isles and Grand Manan, New Brunswick and Digby Neck, Long Island and Brier Island, Nova Scotia areas. Right whales are one of four large whale species commonly observed in the Bay of Fundy. The industry in New Brunswick and Nova Scotia experienced rapid growth in the mid- to late 1990s. In 1998, there were 140,000 individual whale-watching trips in all of New Brunswick and Nova Scotia with $5.12M in direct expenditures (Hoyt 2000). Whale watching is also an important economic industry in Québec with occasional sighting of North Atlantic right whales. The influence of right whale abundance on the overall viability of whale watching tour operators is not currently known as other whale species are also seen. However, the public’s awareness and sensitivity toward right whale issues is enhanced through the educational component of whale watch excursions.
1.6.2 Aboriginal
Right whales were found historically, and continue to be seen in the areas ofMi’kmaq, Passamaquoddy, and Maliseet Aboriginal communities. Harvesting of marine mammals is a tradition of Aboriginal communities in many parts of North America that reaches back several thousand years. Historical and present-day uses and values of the North Atlantic right whale to Aboriginal communities are not known but there are currently no harvests for food, social and ceremonial purposes.
1.6.3 Ecological
Right whales are of great biological and ecological interest since they are one of the few marine mammals of this size that feed so low on the trophic chain (Gaskin 1991). Given this characteristic, right whales may be a good representative of ecosystem health, and a potential useful tool for monitoring the environment, as the species is known to show sensitivity to subtle environmental changes.
The reproductive biology of right whales also makes them an interesting species. Data suggest that right whales represent the most extreme case of sperm competition in the animal kingdom. Understanding how this mating system impacts population dynamics (including the limits it places on growth and reproduction rates) may not only aid our understanding of this species, but could also aid our understanding of reproduction and reproductive biology in general.
This species may also be important for understanding small populations. The long-term data set and extensive genetic data combine to make this animal one of the most well-documented wild species. As such, it has the potential to be a model for understanding the biology of small populations.
1.7 Population Size, Structure and Trends
The population of right whales in the western North Atlantic was estimated in the COSEWIC report (2003) to number about 322 animals. The COSEWIC report did not present the methods used to estimate population size, however the estimate represents the number of catalogued right whales thought to be alive in 2003. Kraus and Rolland (2007) suggest that currently, there are no reliable estimates of right whale population size beyond stating that about 350 animals remain. In terms of trends, the population appeared to have been declining in the 1990s (Caswell et al. 1999, Fujiwara and Caswell 2001, Fujiwara 2002, Caswell and Fujiwara 2004), but only data through 1998 were used in those analyses.
The North Atlantic right whale population is currently assumed to represent one interbreeding population; however this topic has yet to be adequately addressed. It is recognized that there are great differences in the habitat use patterns of different individuals (Brown et al. 2001), and that the population shows significant signs of structuring or fidelity in relation to nursery use by mothers (Malik et al. 1999). However it is not known if these patterns also result in the population being structured into two or more subpopulations separated by some degree of reproductive isolation. Identifying cryptic population structure such as this represents one of the primary concerns in conservation biology because conservation actions often fail in situations where structuring exists but is undetected or unaccounted for (Taylor and Dizon 1999). Testing for structure will identify if this species can be managed as one interbreeding population, or if it represents more than one continuous gene pool, with each sub-population requiring different management considerations. Additionally, these data will reveal previously hidden information on social structure, reproductive biology, and habitat use patterns. The tools necessary for addressing this issue (genetic profiles at a large number of variable loci) are now available, and this work is currently underway.
The North Atlantic right whale population was probably even smaller in the past than it is today (Reeves et al. 1992, Reeves 2001). Malik et al. (1999) found only five matrilines represented in the mitochondrial DNA (mtDNA) from over 200 animals sampled in the western North Atlantic population. Since mtDNA is inherited only from the mother, this suggests that the population went through a very small "bottleneck" at some time in the recent past. It is important to recognize, however, that each mtDNA haplotype could have been represented by more than one female and therefore the study by Malik et al. (1999) does not necessarily imply that only five female North Atlantic right whales existed at some point in the past.
Based on analyses of stranding, entanglement, and photographic data, Kraus (1990) and Kenney and Kraus (1993) estimated that mortality ranged between 5% and 18% during the first three years of life. Moore et al. (2007) determined that calves and juveniles are at significantly greater risk of dying than adults in a given year, though their analysis of all known mortality events shows no particular distribution among age classes or between sexes. Adult mortality rates are very low, probably less than 1% annually, although population modelling studies (Fujiwara and Caswell 2001, Caswell and Fujiwara 2004) indicate that adult female mortality rates are substantially higher, and are the major contribution to the lack of recovery (Fujiwara and Caswell 2001, Kraus 2002).
The North Atlantic right whale population has been subject to significant anthropogenic mortality (Knowlton and Kraus 2001)and has experienced a significant decline in reproductive rates during the 1990s (Kraus et al. 2001, Caswell and Fujiwara 2004). In the period from 1980 to 1992, annual estimates of population size back-calculated from data on calving and mortality indicated a steady increase from 255 in 1986 to 295 in 1992, implying a mean net annual growth rate of 2.5% (Knowlton et al. 1994). Fujiwara and Caswell (2001) however calculated the asymptotic population growth rates from 1980 to 1995 and found that the rate had, in fact, declined from 1980 to 1995. Those authors suggested that if the 1995 growth rate were maintained, the population would become extinct inperhaps as little as 100, and on average about 200 years. This latter analysis is currently thought to best represent the population growth rate trend for that time period (Kraus et al. 2005). The eastern North Atlantic population probably numbers in the low tens of animals, at most, and is certainly too small to offer any hope of a “rescue effect” on the western population.
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