Fine-scale Habitat Use, Abundance, and Size Structure of Mummichog (Fundulus heteroclitus) Within an Estuarine Tidal Creek of Southern New Jersey
This study demonstrates that fine-scale microhabitat characteristics, particularly shallow marsh pools, are stronger predictors of mummichog abundance and size structure than broad seasonal patterns, highlighting the critical role of localized habitat variation in shaping estuarine fish populations.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Estuaries are the brackish meeting places where rivers meet the sea, a zone of constant change where water levels rise and fall with the tides, and where salinity, temperature, and oxygen levels can shift dramatically over short distances. In these dynamic environments, fish must be adaptable to survive, yet they are not equally distributed across every inch of water. Even within a single creek, the bottom might be muddy in one spot and rocky in another, or one area might be sheltered by tall grasses while the next is exposed to open flow. Understanding how fish navigate these fine-scale differences is crucial for protecting coastal ecosystems, especially as human development and climate change alter these fragile landscapes. The mummichog, a small, hardy fish found along the Atlantic coast, is a perfect subject for this kind of study. It is abundant, tolerant of harsh conditions, and plays a vital role in the food web, serving as a meal for larger predators while feeding on smaller invertebrates. Because it is so common and resilient, scientists often use it to gauge the health of an estuary, but knowing exactly where these fish live and why they choose those specific spots requires looking much closer than broad surveys usually allow.
Researchers at Stockton University in New Jersey decided to take a very close look at how mummichogs use the tiny, distinct habitats within a single tidal creek called Nacote Creek, part of the Mullica River-Great Bay Estuary. They set up a long-term monitoring project, placing small, unbaited wire traps in five different locations every week for over a year. These locations were chosen to represent the variety of microhabitats found in the area: a deep channel in the middle of the creek, a creek edge with wooden pilings, a shallow boat basin that is too small for boats to use, a marsh edge lined with tall reeds, and a shallow marsh pool filled with grasses and organic debris. By checking these traps weekly and measuring the length of every fish caught, the team could track not just how many fish were in each spot, but also how the size of the fish changed with the seasons and the specific type of environment.
The results revealed a clear preference for shelter and shallowness. The traps placed in the shallow marsh pool and the shallow boat basin consistently caught the most fish, far outnumbering those in the deeper, more exposed parts of the creek. In fact, the deepest location, a spot in the middle of the main channel, yielded no mummichogs at all during the entire study. This finding suggests that even though mummichogs are known for their ability to survive in tough conditions, they still have a strong preference for specific, sheltered environments. The shallow marsh pool was particularly important, supporting the highest number of fish throughout the year and hosting the largest individuals. The boat basin, despite being a man-made structure, functioned similarly to a natural marsh pool, offering a calm, shallow refuge that the fish clearly favored over the deeper, faster-moving water of the main channel.
Seasonal changes also played a role, but the influence of the habitat itself was stronger. The researchers found that fish were generally larger in the fall, which aligns with the natural growth pattern of young fish that hatch in the spring and grow throughout the summer. However, the size of the fish depended heavily on where they were caught. The largest fish were found in the shallow marsh pool and the boat basin, while the smallest fish were more common along the deeper marsh edges. This indicates that different life stages of the fish may prefer different parts of the creek, with larger fish seeking out the stable, resource-rich environments of the shallow pools. Interestingly, the number of fish caught in the shallow habitats remained high even during the winter, suggesting these areas serve as critical overwintering refuges where the fish can escape the harshest conditions.
The study highlights that the health and distribution of a fish population are not just about the overall temperature or salinity of the water, but about the specific details of the local environment. A difference of a few meters in depth or the presence of a patch of grass can determine whether a fish thrives or avoids an area entirely. The fact that a man-made boat basin could support such a high abundance of fish shows that human structures can sometimes provide valuable habitat if they mimic the shallow, sheltered conditions that these fish need. As coastal areas continue to change, understanding these fine-scale preferences becomes essential. It is not enough to know that a species is tolerant; scientists and managers need to know exactly which micro-habitats are essential for their survival, recruitment, and growth, ensuring that the small, quiet corners of the estuary are preserved alongside the larger, more obvious features.
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