Across Space and Seasons: Patterns in Texas Marine Communities

Figure 1. Map of TPWD CF gulf trawl areas sampled. Inset photo shows TPWD staff sorting the catch from a Gulf trawl sample.

Figure 2. Percent of years where paired areas were significantly different from one another. Area pairs where areas next to one another are indicated with *.

Figure 3. Average number of years for each area before a significant difference in the marine community is detected. Error bars show + 1 standard error for each average value.

It’s a beautiful day and you’re out enjoying our wonderful Texas bays and marine waters when you notice Texas Parks and Wildlife Department (TPWD) Coastal Fisheries staff setting gill nets, pulling trawls, or dredging for oysters. You may wonder what they are doing and why. You most likely saw us collecting data for our long-term fishery-independent monitoring program. Our program is one of the most robust and longest running marine fisheries data sets in the entire world! TPWD collects data using a variety of gear types including gill nets, bag seines, and trawls and uses them to help maintain a sustainable harvest of our Texas fisheries while conserving the biodiversity that makes our Texas waters so special. Anglers usually see these datasets in the form of trends in an individual species over time or across the Texas coast, but TPWD also uses these datasets to help understand patterns in entire marine communities. Often the actions we take affect more than just the species they target; they can affect a wide variety of other species that are part of the same marine community. Some actions might also impact areas of the coast differently than others due to the unique communities present in those areas.

To better understand these communities in our Texas offshore waters, TPWD uses data from multiple gear types. Last month Ryan Easton introduced you to our Gulf SEAMAP bottom longline sampling program, which collects data in nearshore waters off the Texas coast. The study reported this month uses data from our Gulf trawl sampling program. This dataset allows biologists to investigate how these communities vary across space and time. For the Gulf trawl survey program, TPWD collects 16 samples every month in state waters offshore of five areas spread out along the Texas coast: 1) Sabine Lake (SL), 2) Galveston Bay (GB), 3) Matagorda Bay (MB), 4) Corpus Christi Bay (CCB), and 5) lower Laguna Madre (LLM) (Figure 1). We collect 192 total trawl samples in each area every year for a total of 960 trawl samples for the entire Texas coast. After each sample is collected, TPWD staff count and measure the species collected by that trawl (Figure 1). For our analysis, we focused on a group of 20 species that make up 90% of the fish typically caught in these trawls along with some recreationally and commercially important invertebrates like shrimp and crabs. This group included angler favorites like the red snapper along with less commonly targeted fish like Atlantic croaker, Atlantic moonfish, and star drum. Most gulf trawl samples TPWD collects will have several of these species in them, but one sample might have more Atlantic croaker while another might have more white shrimp. The patterns in which species are found in higher or lower numbers gives us a picture of the marine community at the time and place the sample was collected.

As you might expect, adjacent areas often had communities that were similar, but there were exceptions. For the 26 years of Gulf trawl data we examined, the SL and GB areas on the upper coast had significantly different communities in only 12% of the years (Figure 2). MB and CCB on the middle coast had significantly different communities in only 4% of the years. The community differences were larger for GB and MB, which were different from one another in 65% of the years. The communities of CCB and LLM on the lower coast were different 96% of the time. For the entire coast, areas that were not adjacent to each other had different communities in 96-100% of the years we studied.

Our analysis of the patterns in the marine community within each of the five Gulf areas showed that the community structure in each area was likely to be similar between years separated by shorter intervals. For example, the community in GB in 2000 was similar to the community in GB in 2001, 2002, 2003, and 2004. However, the community in GB in 2005 was significantly different than the community in GB during 2000. This type of pattern was typical for most areas over the time period studied, and it took multiple years in most cases before the community in an area shifted significantly. However, the amount of time on average before a community shifted differed substantially between areas of the coast (Figure 3). The SL community on the upper coast did not begin to show a significant change until 7.6 years had passed, on average. Moving down the coast to GB, MB, and CCB, the communities began to differ a little faster than the SL community. These communities began to differ after an average of 5.2, 5.7, and 6.0 years respectively. For LLM community differences were evident after an average of only 3.6 years had passed. This means that on average the LLM community changes more than twice as often as the SL community.

Studying these types of patterns in whole communities along with the trends for individual species helps TPWD understand how to maintain sustainable populations of commonly targeted species that are part of diverse communities. By understanding how communities differ across time and the Texas coast, TPWD can more effectively manage our unique ecosystems and keep the waters of Texas healthy for years to come. And remember life’s better outside!