Research

EFFECTS OF INSHORE TRAWL FISHERIES ON JUVENILE GAG

Stallings Home Page

Like many reef fishes worldwide, gag (Mycteroperca microlepis) populations in the Gulf of Mexico have been intensively targeted by both commercial and recreational fisheries and are showing signs of being overfished. Both the size and age structures of gag populations have declined and the sex ratios of this protogynous hermaphrodite have become increasingly skewed (presently 50 females: 1 male in fished areas; historically 5:1). Proper management of human activities is necessary to ensure long-term sustainability of the gag fisheries and the populations that support it.

Management must address the factors that can affect species throughout their life cycles. Gag undergo several ontogenetic habitat shifts (Figure 1) and knowledge about the impacts of human activities on each stage is necessary for successful management. Juvenile gag settle to seagrass meadows each spring, primarily during April and May, and remain in the seagrass until the following fall egress (usually in October) to offshore reefs. Although it has been well established that juvenile gag extensively utilize seagrass meadows, very little is known about how anthropogenic activities can influence their success at this early life-history stage.

A fishery for bait-shrimp operates in the same nearshore, shallow seagrass meadows that juvenile gag inhabit. The fishery provides live shrimp to recreational fishermen and bait houses and harvests are made on a per-order basis. The fishery uses rollerframe trawls towed for short periods (3 to 15 minutes) over the seagrass. The catch is placed in aerated, flow-through live tanks onboard the vessels and bycatch is separated from the shrimp and released back into the water. Handling and out of water time is minimal for the bycatch. Although mortality of bycatch in the bait-shrimp fishery is probably much lower than other trawl fisheries, delayed mortality of several fishes has been experimentally demonstrated to equal or exceed 20%. These mortality rates may be conservative, however, as they did not include cryptic mortality due to predation from species that follow commercial rollerframe vessels.

My colleagues, Chris Koenig and Felicia Coleman, and I have identified two indirect and potentially detrimental effects that the inshore trawl fishery may have on juvenile gag grouper: 1) bycatch of gag by the fishery and 2) alteration of community structure and abundance of gag prey items. We have begun a project funded by NOAA (Saltonstall-Kennedy) to address these issues by combining field sampling with laboratory experiments. Our work aims to quantify the rate of bycatch of juvenile gag using rollerframe trawling methods and will determine whether the rates are affected by either abundance or size of juvenile gag.

In addition to being inadvertently captured as bycatch, juvenile gag may also be affected by the bait-shrimp fishery via changes to the structure of their prey communities. Food web alterations can have unsuspected effects on fishes, and therefore require a clear understanding for proper management. Our research addresses this problem by first quantifying the rates of capture for their dominant prey species by rollerframe trawls. Diets of juvenile gag grouper primarily consist of penaeid shrimps (~25%), fishes (especially juvenile sparids; ~25%), and hippolytid shrimps (~50%). Both penaeid shrimps and juvenile sparids are captured in large quantities by the bait-shrimp fishery as targeted and bycatch items, respectively, while the smaller hippolytid shrimps do not appear to be captured at all. Repeated fishing efforts in local seagrass meadows may therefore have a tremendous impact on a large portion of gag prey. We are combining our estimates of prey capture rates with laboratory experiments to test for both prey preference and how preference may be changed due to reductions of captured prey items.

This project may also serve as a model system to better understand the ecosystem-level effects of human-induced changes to other food webs. For example, much work is needed to better understand how commercial food-shrimp fisheries in the Gulf of Mexico impact reef fish (e.g., red snapper) fisheries. While previous work has revealed substantially high levels of bycatch in the food-shrimp fishery, experiments to test the effects of altered food webs on reef fishes in these offshore systems would be extremely logistically challenging. The nearshore system in which we are working is far more tractable than offshore systems where these other commercial fisheries operate.