On a sunny early October day, ZNE’s Field Conservation staff conducted a unique survey at an urban park in Boston. Department Director Bryan Windmiller and Field Conservationist John Berkholtz visited a spring-fed pond where the state-Threatened threespine stickleback fish makes its home. This small fish, about 1.5 to 2.5 in long, has three dorsal spines (hence the name) as well as two pelvic spines. This species is primarily a salt and brackish water fish and is common in much of North America, Europe and Asia. However, there are a few permanent freshwater populations. One of these unique freshwater populations is right here in Boston. In fact, it is the southernmost freshwater population known, and the only one of its kind in Massachusetts. At some point in the past, this population got disconnected from populations found in coastal estuaries in the Boston area.
In addition to isolation and freshwater existence, this population exhibits three unique morphological features. These fish have lateral shiny plates along their sides, like plates of armor on a medieval knight. Some individuals are completely plated starting behind the gills along the entire length of the body, some are medium plated along the front half, and some have only a couple of plates. Other morphologies have been found in populations throughout their range making for an interesting study system for environmental adaptation and speciation.
When conducting our survey, we placed minnow traps throughout the pond. Half were baited with a little bread and dog kibble while the other half were not baited. We checked minnow traps the next morning and were pleasantly surprised at the good results! In total, we captured more than 300 fish. Fish were removed and temporarily put in plastic bins for counting and photographing. Though a few more fish were caught in baited traps than in the un-baited ones, the most important factor seemed to be the placement of traps. The majority of the fish we discovered liked to congregate under and alongside a large fallen tree. Though perhaps unsightly to human eyes, this shows how important tree deadfall is in aquatic environments – fish need such structures to hide from predators and forage for food. This phenomenon is also observed in streams managed for reintroduced brook trout, and even the wood turtles we track use such deadfalls.
Another spot where we found a concentration of fish was where spring water was bubbling up from the pond bottom, likely adding to the dissolved oxygen levels. We observed that these fish tend to school together and seemed to be quite hardy, living in a small urban park with wide variations of local climatic conditions. Even with more than 300 fish captured, we have no doubt there were many we did not capture. In the future, we hope to continue to monitor this unique fish and find ways we can safely mark each individual caught in order to get a more accurate estimate of the total population in future capture surveys.