DIVE TIME by Tim Grollimund

Unnatural selection has consequences

The ReporterApril 11, 2014 

A derelict lobster trap on Molasses Reef.


Recently, on one of those flat, no-wind-no-current days, we dove the Bibb. We never dive the Bibb when there is any significant current since the wreck lies on its side.  

That same amount of current is fine for diving the sister ship Duane. On the Duane, there are plenty of places to hide from the current, and of course, the crow’s nest is a highlight of any dive.

On the Bibb, it’s all about the prop. To me, nothing says shipwreck like an exposed prop in 100 feet of water. It is interesting how different the marine population is on these two sites.

The one thing they do have in common is the vast collection of fishing tackle entwined on every part of the wreck, including the mooring lines. On this most recent dive, there was a guy fishing on the other mooring ball, and we watched him get his line hung on the wreck before we jumped in. We saw his lure hooked on the mooring line as we moved toward the prop.

I am always amused when I hear leaders from some user groups say divers do most of the damage in the sanctuary. Perhaps if a few of them jumped in the water and took a look they would see all the debris — in and out of the protected areas — of both trap gear and hook gear. There was a study released in March by NOAA and FWC on the vast amount of trap gear in the sanctuary. See the NOAA/FWC article here:http://timgimages.com/PDF/LobsterTrapDebris.pdf

Figures in this study also include observations from previous studies, documenting 10 to 12 pieces of trap debris per hectare. They estimate the total footprint of trap debris at 60 hectares (the size of Hens and Chickens SPA). When the movement of the debris across the bottom is considered, the damaged area could be as high as 868 hectares. That is the size of the Carysfort, Elbow, French, Molasses, Conch, Davis and Alligator SPAs combined.

Aside from all the derelict gear, there is a mounting pile of evidence showing that fishing pressure can affect the life-cycle development of species. This line of thought first sparked my interest when I saw the photographic study by Loren McClenachan a couple years ago. Take a look at the shrinking sizes from the 1950s to the present — the modern era day’s catch is significantly different than the earlier catches. 

They just don’t make ‘em like they used to. You can see it here: http://timgimages.com/PDF/McClenachan_09.pdf

Based on the refinement of an earlier, simple model, some Australian scientists conclude that a 25-percent evolutionary change in trait values is possible over a period of 30 generations in wild populations. 

Now look at the photos in Dr. McClenachan’s study again. The traits? Body size and age at sexual maturity. Smaller, younger-at-maturity specimens are now the norm in this shifting baseline. This has implications on spawning, as the removal of larger, older animals leaves younger, less productive spawning members of the population to breed.  See this interesting study here: http://timgimages.com/PDF/FishingPressureEvolution.pdf

This also may slow the recovery rate if fishing pressure is reduced or stopped. Another study looked at a wider array of wild populations, emphasizing this aspect: “Nevertheless, it is likely that some undesirable changes observed over time in exploited populations (e.g., reduced body size, earlier sexual maturity, reduced antler size, etc.) are due to selection against desirable phenotypes — a process we call ‘‘unnatural’’ selection. Evolution brought about by human harvest might greatly increase the time required for over-harvested populations to recover once harvest is curtailed because harvesting often creates strong selection differentials, whereas curtailing harvest will often result in less intense selection in the opposing direction. 

We strongly encourage those responsible for managing harvested wild populations to take into account possible selective effects of harvest management and to implement monitoring programs to detect exploitation-induced selection before it seriously impacts viability.”  

See the study here: http://timgimages.com/PDF/HumanInducedEvolution.pdf

Finally, a study of Atlantic Cod — this is perhaps one of the most well known fishery collapses of modern times — is worth the read since it deals with a species that is long-lived and slow to maturity, like many fishes in the snapper-grouper complex.  

Trait changes caused by fishing pressure (smaller size, younger age at maturity) may take 20 to 30 times longer to regain than the time period in which the changes manifested. In other words, past fishing-induced evolutionary (genetic) changes may in fact be the new norm. 

Their main point is full recovery to pre-harvest levels, from a genetic trait perspective, may be very slow, leading to less than expected recovery rates after the initial surge in biomass once fishing pressure is reduced. See the study here: http://timgimages.com/PDF/ImplicationsAtlanticCod.pdf

Once again, we see science that demonstrates the way we are managing our sanctuary    may be leading to an unsustainable ecosystem. All user groups exert pressures that have consequences. While I agree that healthy ecosystems are resilient, how much pressure can be exerted before the resilience is not enough?

We have the opportunity to work together to make some changes. Right now, after the Key Largo meetings in March, I do not see any significant movement towards meeting the sanctuary’s documented objectives of ecosystem-based management. Shame on us.  Generations from now, history will be our judge.

Will we kick the can down the road, or will we develop a managed, sustainable sanctuary ecosystem that makes more cans?

Tim Grollimund is a freelance photographer and PADI divemaster based in Key Largo.  He can be reached at tim@timgimages.com.  Tim is a member of the Florida Keys National Marine Sanctuary Advisory Council.  Opinions expressed by Tim are not the official views of the FKNMS.


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