Dive Time with Tim Grollimund

Porgies play a role in reef studies

December 20, 2013 

A jolthead porgy is easily identified by the orange coloring in the corner of its mouth.


About 20 years ago, I was a volunteer diver on the Scuba Diving Team for the Virginia Aquarium and Marine Science Center. All types of Chesapeake Bay critters filled the aquarium, which was small by today's mega-aquarium standards.  

It was big enough to swim around, play with the animals and put on a show for visitors. I had a great time, and watching the kids' reactions on the other side of the glass was the most entertaining part of the work.

Part of the show included a feeding session. The stingrays would mob me, sitting on my head, waiting for me to dole out the next chunk of squid. I got to hand feed butterfish to the nurse sharks (frowned upon by the staff, however) and generally have a blast hanging out, performing the show and cleaning the inside of the glass. They kept the tank at bay temperature, which is a heck of a lot cooler than our water at the coldest part of the year.

The fish you really had to watch out for were the sheepsheads. They liked getting fed, and were not shy about it. My first feeding experience in the tank left me with a swollen finger from the bite of a sheepshead. If I had not been wearing thick gloves, I am sure my finger would have been badly damaged. And so it is with the porgy family. Most of them are crunchers.

I don't see many sheepsheads down here, but I do see their cousins in the porgy family fairly often in certain places.  They are typically solitary and hang out on the fringe of the sand channels, cruising through to grab small crustaceans out of the sand. I have seen them following stingrays from time to time, but that is not the usual case.

When you get close to them, the are decorative, with colors primarily around the head and lips. According to my REEF fish identification book, there are 16 species in the Caribbean. I have a long way to go to complete that list.  Mostly I see jolthead and saucereye porgies.

There is not an overabundance of studies on porgies, although the red porgy is closely monitored as a fishery. Red porgy catch limits were reduced recently, and the species is considered overfished. This fishery is usually closed to commercial fishing from January through April, but was just closed this month under the new regulatory amendment.

In my research, I did not find much on the jolthead and saucereye species. I did find a thesis by Amanda Tyler-Jedlund from the University of South Florida on littlehead porgies. She points out that within the porgy family, reproductive biology is complex.  

Within the family some species change from male to female (protandry), and other species change from female to male (protogyny), as is the case with littlehead porgies. In her study she found no male specimens in the smaller size classes, but males dominated the larger size classes and older ages. Based on these observations she states that the sexual development patterns of littlehead and red porgy are similar. Why is that important?

Since little is known about the reproductive biology of the porgy family, implications for fisheries and reef ecology enter the discussion. If the larger sizes are dominated by males, and there is a size limit on the taking of fish, it may change the ratio of females to males. The ratio of females to males will fall, prompting more females to change to males at what could be a less-than-optimal age. Examples from other species indicate there are triggers not only from fishing pressure, but also from environmental and behavioral conditions that affect sex reversal.

According to Tyler-Jedlund, models for stock assessment that do not include life history parameters such as sex reversal are not appropriate for estimating monandric sequential protogynous hermaphrodite species, like some porgies and groupers.

This is being addressed now, and recently a workshop was held by the Mid-Atlantic Fisheries Management Council to examine the models. The analysis of red porgy found that "Because fisheries target larger red porgy, which are primarily male, SPR [Spawning Potential Ratio] based on male biomass declines quickly with increased F[ishing] leveling off at a very low value. By contrast, SPR based on female biomass declines much less quickly and levels off at a much higher value.  

Current stock assessments use SPR calculated based on all mature biomass, which has a relationship with F[ishing] intermediate between the two sex-specific calculations." See page 24 of the report here:http://timgimages.com/PDF/Modeling_Workshop.pdf

There is another interesting item that shows the change in the proportion of males on page 6, Table 1. Gag and scamp groupers, for example, showed declines in the percentage of males from 17 percent to 1 percent, and 36 percent to 18 percent, respectively. These are species that form large spawning aggregations. To me, that supports the protection of spawning aggregation sites.

The porgy family demonstrates an important example about how we manage our ecosystem going forward. This one group, which is not as studied as snappers and groupers, reiterates the point that there are many complex variables that should be considered when making ecosystem management decisions. It needs to be examined as a whole, since each part, each species, each decision on size or catch limits, all affect the other kids in the pool.  

Fisheries data, biomass movement, reproductive characteristics and environmental events are all pieces of the ecological puzzle that point to ecosystem-based management that takes a wider view than specific species control regulations.

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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