A recipe for the evolution of smaller fish stocks?

fish face a tradeoff of where to use their energy, much like the polluted fish in the Lorax by Dr. Suess

Overfishing is most often implicated as the cause of decreasing fish stocks and that makes a lot of logical sense if you’ve ever seen a large commercial trawler unload its catch. But there very well might be another force at work in the precipitous decline in fish stocks worldwide: pollution. The basic premise is that it takes resources to deal with pollutants that normally would be given to growth and reproduction. Through polluting the ocean, we have selected for the fish individuals that can most effectively divert those resources, inadvertently also selecting for smaller fish that reproduce less. That has huge implications for the fish’s population dynamics and potentially total fish stock. More details below the fold…

Many fishers see pollution as the elephant in the room that has been left relatively unmanaged in favor of easier-to-enforce rules against overfishing. In North Carolina, at least, this has been documented by Andreatta and Parlier (2010) in terms of the number of regulations addressing possible threats to fish stocks. In addition, the number of possible causes for a decline in fish stocks has never been fully enumerated in one location. But in this particular case, the fishermen may have a point.

It’s been fairly well documented that many fishery species are susceptible to mortality due to pollution. Most recently, you can think of what happened to fish near the BP blowout in the Gulf. The slightly different and arguably more important question is what happens at chronic exposures of lower doses of these chemicals – that is, when we know fish have been exposed and they didn’t die?

Step 1: Select for toxin resistance.

Fish that don’t die as a result of exposure have some degree of tolerance to the chemical to which they’ve been exposed. Therefore, after a pollution event or in areas where pollution is always present, you would expect to find individuals with the capability to withstand the exposure. This is exactly what Gardestrom et al (2008) found in copepod populations exposed to copper pollution – the exposed populations had decreased genetic diversity associated with toxin resistance.  In the long term, however, they didn’t observe a  decrease in population as the surviving genotypes reproduced to fill in the lost population that died as a result of exposure. The copepods hadn’t met the conditions of step 2.

Step 2: Energetic tradeoff between tolerance and reproduction.

“Tolerance” is actually a term that embodies numerous strategies for dealing with toxin exposure. An individual can either passively tolerate the toxin and whatever physiological effects it causes or metabolize the toxin to remove it from the environment. Both strategies can result in decreased energy available for other tasks, such as growth and reproduction, as they require energy either in repair or direct metabolism.

In a review by Medina and Correa (2007), tradeoffs were observed in a large proportion of genotoxicity studies. The state “studies have shown that microevolution due to pollution can have fitness costs associated with the altered physiological processes that enable resistant individuals to cope with the toxic stress”. That means instead of producing the next generation, species are fighting the effects of pollution.

Step 3: Maintain pollutant exposure.

Since we’re talking about long-term effects, the kind of pollutants at hand are not the one-time spills, but instead the constant exposures typified by farm runoff of pesticides, stormwater runoff, and industrial effluents. Medina and Correa also note that part of the tradeoff matrix occuring in species is also resilience against future exposures and stresses – and it appears that organisms by and large can only effectively adapt to one type of exposure. That is, adaptive responses are only adequate to cover the strongest toxin and subsequent stresses will be dealt with differently or mortality will rise. But let’s face it – most water bodies close to human civilization have some level of chronic exposure.

That’s it – 3 steps. Although no one has completely modeled this path, I suspect that such a study is not too far in the future. There are definitely systems that meet the conditions needed for all 3 steps. So perhaps the fishers have a point – they’ll stop fishing when we stop polluting. We all should share the costs of conservation efforts.

This post was inspired by the challenge set forth by NESCent in their blogging contest

~Bluegrass Blue Crab