Paper: Davies, C. E., Johnson, A. F., Wootton, E. C., Greenwood, S. J., Clark, K. F., Vogan, C. L., & Rowley, A. F. (2014). Effects of population density and body size on disease ecology of the European lobster in a temperate marine conservation zone. ICES Journal of Marine Science, 1–11. doi:10.1093/icesjms/fsu237
Marine protected areas (MPAs) were developed relatively recently to help protect biodiversity as well as commercial fishing stocks from being overfished. There are a lot of benefits to the MPA system – whole ecosystems are protected from damaging fishing methods like trawling or dynamite fishing, whole populations are allowed to recover and spread to areas outside the MPA, and whole communities are allowed to recover from human intervention. However, not much research has tested how an MPA can be a bad thing for animals in terms of negative effects of a higher population density (more animals occupying the same amount of space).
While MPAs have the best intentions at heart, increased population density in these areas can lead to some problems. Studies have already shown that increased population density can lead to higher occurrences of disease. Previous studies have found that lobsters in MPAs have a greater chance of contracting shell disease. This study sought to investigate that further, as well as look for other diseases, such as gaffkaemia (potentially fatal bacteria) and white spot syndrome (viral disease). All of these diseases are present in the European lobster Homarus gammarus, the focal animal in this study, and are more prevalent in injured lobsters.
Lobster shell disease has been a hot topic recently because it has contributed to the decline of American lobster fisheries – for more information, see this great oceanbites article. There are two types of shell disease – epizootic, which is the disease harming American lobsters, and enzootic, the disease studied here. The enzootic type of shell disease is less devastating in scope than the epizootic type, but both disease erode the lobster’s shell and can be fatal.
Disease spreads in lobsters (and most other animals, too) depending on a variety of factors such as stress. Stress can be induced by higher water temperatures as well as higher population density. More lobsters in the same vicinity means there will be more lobsters coming into contact with one another, so the probability of meeting one with disease is increased, as is the probability that the lobsters will engage in a fight and become injured.
The researchers in this study conducted their research in a marine protected area in the UK called Lundy Island. This location was ideal for their study because it has two levels: the restricted zone (RZ), where lobsters can be caught in pots only, and the no-take zone (NTZ), where lobsters cannot be caught at all. They wanted to find out if lobsters in the NTZ were more likely to have diseases than their counterparts in the RZ, and in that analysis, they also looked at what kinds of lobsters in both areas were more likely to exhibit disease.
They surveyed populations using traditional lobster traps, recording the length, level of disease, sex, and any injuries. They also took blood from each lobster in order to run a sort of “tox screen” on it – they used the blood in conjunction with genetic techniques to identify which pathogens were present.
The researchers’ catch data revealed differences between the populations of the NTZ and the RZ. In the NTZ, they caught significantly more lobsters than the RZ and their catch per unit effort (the amount of lobsters caught in each trap) in the NTZ was twice that of the RZ (Figure 1).
They also found that the lobsters in the NTZ were significantly larger than those in the RZ. These two findings together suggest that the MPA is doing its job – there are more lobsters, and those lobsters are larger, in the area that does not allow any fish or lobsters to be removed from its waters (Figure 2).
When the researchers looked at shell disease, they found that 28% of the lobsters in the NTZ were infected, compared with just 17% in the RZ. The most important factors in determining whether a lobster had shell disease were sex, injury, and size. According to mathematical models calculated from their data, a male lobster was 83% more likely to have shell disease; an injured lobster was 76% more likely to have shell disease; and a larger lobster was 83% more likely to have shell disease. Because the lobsters in the NTZ were larger, the result that more of them have shell disease makes sense. The larger a lobster gets, the less it molts (sheds its exoskeleton to grow a new, larger one). The less a lobster molts, the more likely it is to have injuries that allow for shell disease or allow the disease more time to incubate in the shell.
The researchers also found a higher prevalence of injured lobsters in the no-take zone – 41% versus 19% in the restricted zone. The two most important factors determining how likely a lobster was to be injured were size and site, meaning that a lobster was 71% more likely to be injured if it was large and also 71% more likely to be injured if it was caught in the NTZ (Figure 3).
The other two diseases investigated – gaffkaemia and white spot syndrome – were found to be present in negligible amounts in both sections of the MPA – good news for the lobsters!
The establishment of marine protected areas has been a great thing for a lot of ecosystems – they have helped to restore overfished populations and protect many communities of animals. However, there are other impacts of MPAs that we don’t understand fully. Population density in this particular MPA led to a higher prevalence of shell disease in lobsters. The cessation of fishing for the past 8 years has allowed the lobster population to grow, but that growth is associated with higher levels of disease because the lobsters are larger, more likely to injure each other, and more likely to spread disease to their fellow lobsters in close quarters.
This study illustrates the need to keep an eye on MPAs both before and after they’re set up – even the best intentions have some associated drawbacks. With a stable, long term monitoring system for these areas, scientists would be able to better advise governing bodies and managers on the implementation of marine protected areas.
Hi and welcome to oceanbites! I recently finished my master’s degree at URI, focusing on lobsters and how they respond metabolically to ocean acidification projections. I did my undergrad at Boston University and majored in English and Marine Sciences – a weird combination, but a scientist also has to be a good writer! When I’m not researching, I’m cooking or going for a run or kicking butt at trivia competitions. Check me out on Twitter @glassysquid for more ocean and climate change related conversation!