Age and Growth of Coastal Spotted Seatrout in Texas

One of the main hurdles facing fishery biologists is that the subject of their science occurs under water.  For this reason, fishery biologists are prone to take data points from wherever they can get them.  A great example of this is the use of otoliths to determine age in fishes.  Otoliths are hardened bone structures inside the head of a fish.  They are analogous to inner ear structures in mammals, and perform similar sensory functions.  More importantly for biologists, otoliths tend to deposit rings on a yearly basis, called “annuli”, which when examined microscopically, allow fishery biologists to determine the age of a fish accurately.  Age is a very important data point, and can be related to an individual’s overall growth, mortality, reproduction, and other important biological metrics.  For this reason, a significant component of the research that is conducted by the Texas Parks and Wildlife Department’s Coastal Fisheries division is focused on analysis of otoliths from various coastal finfish species.

To this end, we collected otoliths from Spotted Seatrout (a.k.a. “speckled trout”) during a 20-year span between 1998 – 2018, in an effort to learn more about age structure and growth of this species across the Texas coast.  Over that span, over 14,000 otoliths were collected and analyzed, and collections were made in every bay system in Texas from the Laguna Madre to Sabine Lake.  This study has yielded some interesting results and has broadened our understanding of the biology of this species in Texas.

First and foremost, when we compared body size to age in Spotted Seatrout, we found that females tended to grow faster for a given age, and obtain a larger terminal length (the size at which a typical fish stops growing in length) than males.  This probably comes as no surprise to the seasoned angler in Texas, and frankly it wasn’t a surprise to our biologists either.  Trophy-sized Spotted Seatrout (> 25 inches) are usually females.  As it turns out, the terminal length of female trout in Texas is around 30 inches, compared to about 23 inches in males.  Terminal length in both genders is approached between 8 - 10 years old.  The fact that a “typical” female stops growing at about 30 inches puts the current state record (37¼ inches) into an impressive context and underscores the fact that there is a lot of individual variation that occurs around the population average.

A second interesting finding of our work was that growth in both genders tends to be highest along the upper coast (Sabine Lake and Galveston Bay), and decreases as you move south.  In other words, a 2-year-old fish will be slightly larger on the upper part of the coast than on the lower part of the coast.  The differences were subtle, and there were some exceptions to the trend, but overall fish experience faster growth in our northern latitudes.  This might be due to higher density of forage species, more tolerable water conditions, more suitable habitat, or any combination of these factors.

At this point, you might now be asking yourself, if fish grow faster on the upper coast, how does one account for the fact that places like the Laguna Madre and Baffin Bay are considered trophy fisheries for this species?  Well, a third interesting point that arose from our age data was that fish on the lower coast tend to grow to an older age than fish on the upper coast.  Stated another way, body size is a function of growth rate, nutrition, and perhaps most importantly for Spotted Seatrout, age.  The average age of a Spotted Seatrout in the Laguna Madre (both the upper and lower arms) is around 4 years, compared to less than 3 years in Sabine Lake and Galveston Bay.  This result, combined with our growth rate findings, suggest that trophy fisheries on our southern coast may not be driven by faster growth necessarily, but by an overall older population of fish existing in the Laguna.  Natural mortality (from predators, senescence, etc.) and fishing mortality are both higher as you move up the coast, and this has resulted in spatial variation in the age structure of Spotted Seatrout populations from north to south.

Combined with previous genetic research in our lab, these findings underscore the fact that there is a lot of spatial variability in the biology of Spotted Seatrout in Texas; so much so that we might consider the northern, central, and southern coasts to be functioning as somewhat independent populations.  This regionalization of Spotted Seatrout populations has been realized through regionalization of fishery management regulations through time (for instance, historically regionalized bag limits in the Laguna Madre).  Our 20 years of otolith collections have helped us understand how key biological parameters might be changing through time and across the Texas seascape for Spotted Seatrout, and will ultimately be used to help us design future fishery management regulations that are anchored tightly to the biology of this extremely important Texas sportfish species.