​Managing wildlife in a changing environment: the case of the Baltic grey seal

For most long-lived species, population growth happens over the course of decades, or even centuries. As a result, the way in which we manage wildlife now will set the stage for future generations.
The Baltic grey seal (Halichoerus grypus grypus) is a unique subspecies found throughout the Baltic Sea. Unlike most other seal species, they can give birth on both land and ice, although pups born on ice have a greater chance of survival. In the 1900s, the population crashed from approximately 100 000 individuals to as few as 5 000 as the result of overhunting and sterility caused by pollution. The species is now recognised for its importance as the largest predator in the Baltic Sea. Conservation efforts, beginning in the 1980s, have led to the gradual recovery of the population to more than 55 000 individuals. The Baltic ecosystem, however, has changed significantly over the past 100 years. Industrial fishing has led to the collapse of fish stocks, while the average area of sea ice shrinks due to climate change.

 

 

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In 2020, following decades of protection, Sweden introduced licenced hunting for Baltic grey seals with an annual quota of 2 000 individuals. Combined with quotas from Finland and Åland, this led to a total hunting quota of approximately 3 500. Given the current state of the Baltic ecosystem, are these quotas sustainable? Answering this question required understanding long-term trends in grey seal population growth and digging into more than 120 years of monitoring data.
Luckily, the Baltic grey seal is a well studied species due to its use as an indicator of ecosystem health. This meant we had access to decades of carefully recorded data on population size and health that represented several careers worth of work. Along with collaborators at the Swedish Museum of Natural History, we began to compile data on the Baltic grey seal life history, population trends, and hunting. Based on detailed bounty statistics kept during the early 1900s, we tracked the collapse of the population in the previous century. From annual aerial surveys, which occur throughout the Baltic, we observed the slow recovery of the population to its current levels. Using reports from the necropsies of close to 1 000 animals, we also estimated fertility and survival rates. 
Using all this information, we built a population model which simulated the growth of the Baltic grey seal over the past two decades. We then took this a step further by “reintroducing” hunted animals into the model to explore the effect that hunting has had on population growth over the past decade. This gave us a baseline “undisturbed” population which we could use to explore the relative impact of different management strategies.
Next, we projected population growth into the future, exploring what might happen given current hunting quotas and a range of other management options. We also simulated the consequences of the loss of sea ice due to climate change in the future by incorporating a lower pup survival rate into the model. This allowed us to explore the influence of multiple stressors on population viability.
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

We found that hunting of 3 500 individuals each year, representing the quota for the whole Baltic in 2020, led to population collapse within three decades in all simulations. It was clear from our analysis that the current grey seal hunting quotas in the Baltic were unsustainable. Although the total number of hunted animals have been less than annual quotas over recent years, we recommended that a precautionary approach be taken to management and the assumption be made that quotas might be filled. We found that the maximum number of hunted animals which allowed the population to continue its recovery towards natural abundance was 1 900, but that any further environmental changes might lower this number.
In our simulations, climate change was not predicted to have large impact on population growth in the absence of hunting. When combined with hunting, however, climate change increased the risk of population decline. Our modelling included a simplistic effect of climate change as reduced ice coverage, however there are many unknowns in how grey seals will respond to their changing environment, meaning further field work, data collection, and analysis is needed.
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Now, more than ever, we understand the harm caused by overexploitation, yet we continue to ignore warning signs of environmental degradation in favour of profit. The Baltic grey seal suffers alongside many other species from the effects of industrial fishing. We have optimistically assumed that the population will continue to recover, however this requires a healthy ecosystem to happen. The data and modelling we have compiled should act as a starting point for future work exploring the drivers of population growth in a changing environment.
Understanding the population dynamics of long-lived species takes decades of data. Management decisions in contrast are often made based on short-lived political attitudes and limited information about the role of top predators in the complex marine food web. The history of the Baltic grey seal parallels that of many other large mammal species, presenting examples of both mismanagement and successful conservation. For the sake of maintaining this species in our ecosystem, we have the obligation to take a long-term view of its management.

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Click here to try out our interactive web app for modelling Baltic grey seal population dynamics