The Great Goliath

Today’s feature on Goliath Grouper comes to us from the Gulf of Mexico Fishery Management Council. One of it’s most interesting findings is that Goliath’s, contrary to popular angler opinion, eat very few snappers and other grouper species.
from The Fishing Wire

The goliath grouper population is showing signs of recovery in the Southeastern United States. This should be fantastic news, since goliath are considered critically endangered throughout most of the world, but many Gulf fishermen don’t see it that way.

That’s because goliath grouper have a bad habit of eating fish that are struggling at the end of fishing lines and spears.

Understandably, this opportunistic behavior has led many fishermen to believe that goliath are a nuisance – directly competing with them and decimating game fish populations.

However, scientific studies have concluded that goliath grouper are not the reef fish gobbling, top predators they appear to be, and they don’t seem to harm the snapper or grouper populations on reefs where they live and eat.

How do we know this?

A very straightforward way to figure out what something is eating is to check out what’s in it’s stomach. When scientists capture goliath their stomachs are emptied and the contents are classified.

Using gut content analysis, scientists Chris Koenig and Felicia Coleman (2009) found that, rather than eating all the game fish in sight, 72% of a goliath groupers diet is invertebrates – 62% of which is crabs. The figure below shows how many of each prey type were present in the stomachs of the 226 adult and juvenile goliath grouper studied.

The only downside to gut content analysis is that it only shows a snapshot of what’s been eaten recently. Since we are what we eat, stable isotope analysis can determine what a marine organism has eaten over a long period of time.

Chemical signatures found in the muscles of fish can indicate what food source was used to build that tissue. The more nitrogen in the muscle, the higher up on the food chain an animal feeds. Koenig and Coleman (2009) found that goliath grouper holds a relatively low position on the food chain and actually about even with pin fish.

Finally, in reef fish surveys conducted in southwest Florida, Koenig and Coleman (2009) found that there was no significant relationship between the number of adult goliath and the number of other groupers on a site. Interestingly, the number of snapper actually increased along with the number of goliath grouper on a reef.

So what do we do when the science says something that seemingly contradicts what fishermen are experiencing?

Fishery managers do their best to balance the views and needs of fishermen with the requirements of the law and the science. In 2012 the Gulf and South Atlantic Councils joined up to consider how to move the goliath grouper fishery beyond moratorium. The Ad Hoc Goliath Grouper Steering Committee was formed as a direct result of public input suggesting we consider allowing for a limited fishery as the goliath population recovers.

At the initial meeting, the group recognized the importance of both scientific and public viewpoints for future management of goliath by initiating a science workshop to gather all the research that has been done on goliath since the last stock assessment, and asking for a stakeholder survey and workshop to be conducted.

In January, the Goliath Grouper Steering Committee will hear the results of the science workshop and stakeholder survey.

They are expected to consider how to move forward with potential management actions for the future. More information about this meeting can be found on page 4.

Koenig, C.C., and F.C. Coleman. 2009. Population density, demographics, and predation effects of adult goliath grouper (Epinephelus itajara). Final Report to NOAA MARFIN for Project NA05NMF4540045