North Atlantic right whales are one of the most endangered whale species in the world. Their past is plagued by a difficult history of whaling and they are still facing an onslaught of modern threats from human activities including vessel strikes and entanglement in fishing gear. After commercial whaling ended, southern right whale numbers quickly rebounded and there are likely more than 4,000 whales in the Southwest Atlantic alone. This is in stark contrast to the 366 North Atlantic right whales that remain today. This study sequenced the genomes of right whales to investigate their past and understand the long-term history of these species.
We detected a large decline in the effective population size of both right whale species likely due to whaling, but for thousands of years prior, North Atlantic right whales had much lower effective population sizes than their Southern counterparts in the Southwest Atlantic. These drastically different histories, likely mean the two species have different futures and therefore comparing the poor recovery of North Atlantic right whales to the success of southern right whales is not appropriate.
Long term small effective population size, combined with evidence of recent inbreeding, and current low levels of genetic diversity are a great cause of concern for North Atlantic right whales. Other species have recovered from similar fates, so while the situation is dire, it is surmountable. The best chance the species has for recovery and long-term success is retaining as much diversity as possible, underscoring the importance of modern conservation measures.
Small population sizes increase the chances of mating with close relatives, known as inbreeding, which as we know from humans can cause a number of health problems. Small population size can also lead to a decrease in genetic diversity over time. Genetic diversity is extremely important for a population to be able to withstand and adapt to changing environments. Long-term small effective population size increases a species chance of going extinct, because of these factors. Important follow up research is looking into how this long-term small size has affected the genetic diversity in the species and what that means for their recovery.
No, there are many examples of species that have recovered from population crashes and from having small long-term population size such as the southern elephant seal, the Channel Island foxes, the Chatham Island black robin and the kākāpō. In fact, long term small population size may have provided some benefits; in some cases, species that exist in small populations for generations have been able to remove harmful mutations from their gene pool. More research is on-going to understand if this has occurred in North Atlantic right whales and what this means for their future. Based on this study, and plenty of examples, there is no reason to believe North Atlantic right whales would be unable to recover if anthropogenic threats were removed.
Effective population size is not the same as census size and therefore does not represent the number of individuals alive at any given time point. Effective population size is a measure used in genetic studies that roughly represents the number of breeding individuals in a population, but is influenced by a number of other factors. In most cases, the effective population size is lower than census size, but a direct comparison between the two is very difficult. Therefore, our results are not intended to estimate exactly how many North Atlantic right whales there were at different points in history, but rather to compare the long-term population history of the two species and understand how their history can help shape their future.
Effective population size takes into account the amount of genetic diversity that exists in the population. In a bottleneck event, where a population’s size is drastically reduced, the effective population size also decreases, but as the population size begins to grow in numbers, there is a delay before this growth will be reflected in the effective population size. In time, we will expect to see the effective population size of southern right whales grow. It is also important to note our assessment of Southern right whales is focused on the population in the Southwest Atlantic off the coast of Argentina.
When we estimate changes in effective population size over time, we need to include different parameters such as how fast DNA mutates in a given species, the approximate generation time for the species etc. All of these parameters influence the time scale in our results. We used our best estimates for these parameters based on a lot of other research, but the true value of these factors for our dataset may be slightly different. Because of these estimations, we cannot use our results to exactly pinpoint the exact year whaling began or ended, rather we use them to detect long-term patterns. The fact that the trends we find in our dataset mimic the patterns we expect to find with whaling however, suggest the parameters we chose were very good estimates of the true values.
This study found that North Atlantic right whales and Southern right whales population sizes have been vastly different since the species diverged. These different histories suggest the two species will continue to forge their own paths and as such, we should not use the population recovery in Southern right whales to set goals or benchmarks for North Atlantic right whales. While we cannot make direct comparisons between effective population size and census size, it is unlikely to expect North Atlantic right whale numbers to reach the sizes we see in the Southern Oceans – or even in the Southwest Atlantic. Successful recovery for North Atlantic right whales will require minimizing anthropogenic threats, and decades of steady population growth.