More Than Meets the Eye...

Brian Bartram | Fisheries Biologist | Aransas Bay Ecosystem Field Station, Rockport
More Than Meets the Eye...
We've all heard the clich, "A picture is worth a thousand words." In the fisheries world, this mantra holds especially true for graphs depicting fisheries data. A graph doesn't quite have the same "wow factor" as a picture of a tailing red drum or an angler with a rod doubled over. But a graph can tell you a lot about what's going on with a fish population, albeit with a discriminating eye and an understanding of what that bouncy line that resembles an EKG readout really means.

Many of you may have been to public meetings held by the Texas Parks Wildlife Division (TPWD) in which a graph is displayed on the screen during the course of a presentation. Sometimes the interpretation is fairly straight forward and the conclusion is undeniable. For instance, say the graph presents numbers of a sportfish caught in gill nets over the past couple of decades and the trend line slopes downward, we likely have an issue that needs to be addressed. On the other hand, if we observe commercial landings of a particular species taking a precipitous drop, yet TPWD gill nets are jam packed with them; this may require a little more thought as to what is really happening.

Part of our job as TPWD biologists is to collect, compile, analyze, and review fisheries related data. A superficial glance at a graph can cause alarm and raise a multitude of questions. A longer look at the graph can yield answers, or more questions. A countless number of factors can affect that little line that we see on a graph and the peaks and valleys that we see on that line can sometimes be linked to significant events. Fisheries data can exhibit a great deal of variability, and for good reason. The environments in which aquatic organisms live are often very dynamic. Fish and shellfish can be affected directly or indirectly by drought, torrential rains, or harmful algal blooms (e.g. red tide, brown tide). In Texas, the impacts of major freeze events and outbreaks of red tide can be seen in the population data for many species. Some of these impacts can be seen on a coast-wide basis, others on a more localized scale depending on the spatial extent of the event. Hydrological parameters such as salinity, water temperature, and dissolved oxygen can all influence the health of a fishery, but factors that have nothing to do with Mother Nature can exert an equal if not greater influence. Effectiveness of modern angling techniques influences how many fish make it home to the fillet table. Even something as abstract as angler mentality and behavior can affect catch rates, which can manifest in our data.

Referring to the example mentioned above, I recently found myself perplexed as I looked at some commercial landings data of black drum in Aransas Bay. The landings decreased drastically at one point on the graph, yet as I compared that same time frame within our TPWD sampling data, the same decrease was not apparent (refer to red arrows on graph, years 2000-2001 and 2006-present). I consulted with my supervisor as to the cause and I discovered that the reason was surprisingly simple; the commercial fishermen that were responsible for the majority of the black drum landings have simply moved away to other locations. Fewer commercial fishermen equals fewer fish being brought back to local fish markets. This was a striking example of how fisheries dependent data (reported by both recreational and commercial fishermen) and fisheries independent data (data that come from sampling the resource directly) can differ. There are a number of unique factors that can affect each type of data. TPWD uses both types of data to better understand and manage the resource.

To some people, graphs are boring. To fisheries biologists, they offer insight into what is happening in our bays and offshore waters. It is our job to gather and synthesize data with a trained eye and interpret that data for you, the angler, so that all of us might better understand what is going on beneath the surface.

These graphs allow comparison of fisheries dependent and fisheries independent data on black drum from Aransas Bay. Significant events to note include the major freeze events in the 1980's (red lines) and an increase in commercial fishing efforts for black drum in the mid 90's and early 2000's (black lines). The red arrows on the first graph point to dramatic decreases in commercial landings of black drum. During the same time period, we see an overall increase in gillnet catches of drum. This would indicate the reduction in landings was likely due to a reduction in commercial fishing effort, not a reduction in the overall abundance of black drum in Aransas Bay. Next time you find yourself at a Coastal Fisheries public meeting and a graph is shown on the projection screen maybe there is more to it than meets the eye.