Hidden side effects of MPAs?

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Why the negative spin on a consequence which anybody with even a basic understanding of population ecology would accept and even welcome as inevitable?

Density dependence is a well recognised central tenet of population ecology, i.e. as the density of a population is restored back to unexploited levels, a number of 'natural' trends will increase, such as increased prevalence of disease amongst more crowded populations and older 'senile' individuals (as natural age structure is restored), along with increased competition for space, sexual partners, food, etc., leading to increased fighting related injuries. Per capita production will also decrease due to competition for food, cannibalism, etc. This is naturally what happens when you stop thinning a population through harvesting. It certainly is not hidden or unexpected, nor is it a threat to a successful marine conservation story. It is simply what should be expected to happen when a population is restored back to natural levels.

The negative spin related to findings such as this are eagerly seized upon by the fishing industry and some fisheries scientists [1]  as arguments against no-take MPAs, when they should be more positively presented as a natural consequence of the recovery of populations back to unexploited densities and age structures, along with related spill-over and export benefits. I have been presented with a partial understanding of such findings by several fishermen as yet another reason why no-take MPAs are a bad idea. It would be better if these findings could be presented in a more balanced way that recognises that they are merely a representation of basic population ecology associated with the recovery of marine populations back to natural unexploited levels, rather than as the negative 'rarely broadcast'  threatening and hidden side effects of no-take MPAsWhy the negative spin on a consequence which anybody with even a basic understanding of population ecology would accept and even welcome as inevitable?

[1] Jones P.J.S. (2007) Point of View - Arguments for conventional fisheries management and against no-take marine protected areas: only half of the story? Reviews in Fish Biology and Fisheries 17(1), 31-43. doi:10.1007/s11160-006-9016-8 - Copy of paper. See also pp.46-55 on Divergent views and the quest for common ground amongst fisheries scientists and marine ecologists in Jones P.J.S. (2014)Governing Marine Protected Areas: resilience through diversity (see previous MPA News interview on this book)

Hi Peter,

Thanks for your comment, however, I would say there is no 'negative spin' here - just stating we have found. As mentioned above, and in the paper, there were definitely positive effects of the MPA. 

Lobsters in the un-fished area (No take zone; NTZ) were significantly more abundant, and larger than those in the fished area - a great, positive effect of the MPA. The ban on fishing has allowed populations to increase and illustrates why MPAs are being used. Great! There is also some indication that the lobsters may be moving out of the NTZ into the adjacent fishery - the abundance of lobsters was slightly less in the NTZ than that from a survey of the previous year. Again, this may be a great positive effect of NTZs; stocking fisheries, recruitment etc.

Due to the way the Lundy MPA was created, there was no baseline monitoring therefore we do not have any total pre-NTZ data in order to compare to, so for us, the restoration back to 'natural' population levels was hard to discern. For this reason, in our study, we used what we called the 'refuge zone' as a reference, or 'control' area - this area is fished (but not by trawlers), and showed a significantly lower abundance of lobsters.

However, we were also there to monitor disease. A previous study of Lundy had called for a 'Cost -benefit review of marine reserves' therefore we wished to investigate this further.

We found that if a lobster was caught in the NTZ it was 71% more likely to be injured than a lobster in the fished area (we think this is an artifact of overcrowding from the increased abundance, which leads to competition between lobsters, which are solitary and can be aggressive, so will fight for space, food, mates etc.)

We also found that if a lobster was injured, it was 76% more likely to have shell disease than an uninjured animal, regardless of where it was caught. This makes sense; a lobsters first line of defense is its hard exoskeleton (or cuticle), therefore for a pathogen to enter, the cuticle must be breached somehow.

You must admit, the 'rarely broadcast' statement is very true - It is very rare that you find a study which looks at impacts of MPAs other than population abundance and species richness (all positive effects). I am certainly not against MPAs  - our take home message is that there are other factors which can affect populations such as disease, which must be monitored. It seems NTZs are doing their job, but may need to be re-assessed after a certain timespan… in our case, adult lobsters do not have many natural predators, which is why I suggest controlled fishing as a possible solution to prevent disease caused by overcrowding. 

I hope this clears a few things up, and shows that our study is objective; showing multiple effects of MPAs.

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