Dive Time with Tim Grollimund

How can chubs help science?

January 17, 2014 

Chubs are common on the reef, and are glazed with yellow around the mouth and eyes.


The diving this winter has been a bit spotty for me. Little pockets of a few days here and there between the gusts of wind that last for days upon end.  

Even with what we consider winter, I was reminded last week of how good we had it here while people north of us were experiencing the recent arctic vortex. Some friends of mine from Texas and Nebraska were visiting and were happy just to be in shorts and flip-flops, even though is was too windy to be on the water on the oceanside.

While I was researching the red hind protected area success data for the last column, I saw a newly released paper on chubs from Dr. Richard Nemeth at the University of the Virgin Islands. He was kind enough to send me an author's copy (newly released scientific papers can be very expensive on the research websites).  

I have not seen much written about chubs in all the papers I have run across in the last few years while I have been home-schooling myself on marine biology and science. Believe me, I am quite thankful for electronic storage versus paper storage.

Chubs? They are just kind of "there" under the boat, hanging out, loving those rough water days when people like to feed them.

Like our weather sometimes, chubs do not get much attention. Last year when I was writing the "Sanctuary Science" series, I saw a lot of references to spawning aggregation sites being "multiple species sites." As it turns out, chubs are a great example of a species that hones in on the geophysical aspects of underwater formations for their spawning activity.

To understand this, I first had to look at the characteristics of chubs. They have relatively well-defined home ranges, according to a study in St. Lucia by Eristhee and Oxenford. They acoustically tagged fish in two separate reserves, with different reef shapes to see how the chubs moved. 

They wanted to find out the size of the home range, the use of space within the range and the extent to which the animals went in and out of the reserve. Chubs have favorite spots at different times of the day, and one site for sleeping. They prefer areas with more algal cover and higher rugosity.

The home ranges of the tagged chubs were 0.012 and 0.015 square miles (about 3 hectares). As a comparison, the area of the Molasses Reef SPA is 88.6 hectares. You can see the paper here:  http://timgimages.com/PDF/Chub_Home_Range.pdf

Keep that in mind as we blend the home range study in with Dr. Nemeth's paper. He found chubs have similar spawning characteristics to some grouper species, and use the same sites. Chubs are the only herbivorous species classified as a transient aggregating species. They spawn in the late afternoon and evening after the full moon in January, February and March. This is a similar timeframe to red hind, Nassau grouper and yellowfin grouper, when annual water temperature are at their lowest.  

In fact, federal waters are closed to harvest of selected shallow water grouper species until May 1 (gag, black, red, scamp, rock hind, red hind, coney, graysby, yellowfin and yellowmouth).

According to Dr. Nemeth, "Chubs are rarely observed on the Grammanik Bank or other offshore reefs in the U.S. Virgin Islands except during the three-month spawning season suggesting that they undergo migrations from nearshore reefs to offshore spawning sites. Migrating to shelf edge spawning sites may facilitate advection of larvae into nursery habitats consisting of floating sargassum mats... and would be advantageous for larval survival into the post larval phase which is not dependant on settlement into nearshore habitats."

Here is how I think we can apply this knowledge to the Keys: We know the peak chub spawning activity dates. We know their home range is relatively well-defined. We also know chubs use sites other species use, such as the aforementioned groupers. If we can tag and follow chubs during these months to aggregation sites, could that lead us to previously undocumented snapper and grouper aggregation sites?  

Can we, in effect, use the chub activity as a proxy for spatial and temporal examination of reef segments to discover reproductive aggregations of other species? And if so, how do we go about protecting those vital areas?

More importantly, how do we fund the science to gather the data so we can make an informed decision? This is a long-term view that, in my opinion, could be incorporated in the research plan for the sanctuary.

Let's go back to Goliaths for a second. Their spawning activity came back after protection was put in place. That was also observed with the red hind fishery in the USVI. Both of those are success stories. It seems that an intuitive approach for us is to protect any aggregation sites we can find -- strictly at first, and perhaps less strictly as the numbers increase and the adequate spawning potential ratios are achieved.

That might seem like an oversimplified approach, but so was Occums's Razor, which can be traced back to Ptolemy: "We consider it a good principle to explain the phenomena by the simplest hypothesis possible." As I've said before, I'm a simple guy, and this looks like a simple, verifiable and potentially productive approach.

As a quick update, you may or may not have heard Goliath groupers will remain protected.  See Kevin Wadlow's article about the meeting here: http://www.keysnet.com/2014/01/11/493870/regulators-say-no-for-now-to-opening.html.

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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