Other Dissertation Findings

Here are some other components of my dissertation that did not directly address effects of shoreline modifications on fish ecology. 

Context-dependent behaviors:

Fish are able to make decisions based on real-time assessments of their environments. Oftentimes this means assessing their risk of predation, and fish behave in ways that balance the risk of predation with some other benefit (e.g., feeding). One of the most prevalent ways fish are thought to do this is by schooling. In larger groups, fish are numerically less likely to be eaten (if a fish does get eaten, it probably won’t be them), more vigilant for predators (many eyes scanning for predators), and large schools may disorient predators or prevent them from focusing on one fish to attack. However, swimming in a large school also carries a cost in that fish have to compete with other individuals for resources such as food. So, fish should benefit from schooling in groups that reflect their predation risk environments — why compete with schoolmates for food if there’s little chance of being eaten? While context-dependent schooling (and tradeoffs in behavior related to predation risk in general) has interested people for a while, this behavior has mostly been examined in controlled settings and there is limited documentation of these behaviors in open water.

In this study, I pooled my lab’s snorkel and scuba surveys over the past decade and compared the school sizes of fish in different situations.

First, I looked at deep-bodied, darker-colored fish, and compared the school sizes of fish to the portion of the water column they occupied. These fish are probably safest when they are closest to the bottom where they can camouflage and hide in rocks and algae. I found that these fish schooled in larger groups when they were away from the bottom and probably at greater risk of being seen by a predator. Vantage points high in the water column may enable these fish to find food more easily, so schooling may allow them to offset the risk of occupying waters that are more dangerous but offer better foraging opportunities.

groupsizewcp
Bottom-oriented fish schooled in larger groups when they occurred in the middle of the water column.

Next, I compared the school size of fish that occurred at the surface to the depth of the water occupied. These fish are probably in greater danger when they are away from shallows where larger predators are ineffective and where there are multiple attack points for predators on the bottom to see them at the surface. I found that these fish schooled in larger groups in deeper water, which may allow them to offset predation risk when they are in deeper, and therefore more expansive (more overall prey available), foraging arenas.

groupsizevsdepth
Surface-oriented fish schooled in larger groups in deeper water

Finally, I compared the size of fish to the size of the schools they occurred in. Larger fish are generally safer because they’ve outgrown the jaws of more potential predators and because they can swim away more quickly. I found that smaller fish schooled in larger groups, potentially because they are in greater danger of being eaten.

fishsizevsgroupsize
Smaller fish schooled in larger groups

Relevant publication:

Munsch SH, Cordell JR, Toft JD. In Press. Fine-scale habitat use & behavior of a nearshore fish community: habitat partitioning, nursery functions, & avoiding predation. Marine Ecology Progress Series doi: 10.3354/meps11862. Feature Article

Inferring temporal feeding patterns from behavior vs. diets:

Daily feeding patterns in fish are important to understand because they often reflect ecological tradeoffs (e.g., it’s easier for visually-feeding fish to feed when it’s bright, but it’s also easy for their predators to feed them) or constraints (e.g., digestive capacity) to feeding. Temporal patterns in feeding are typically assessed by examining stomach fullness over time. However, this method prevents us from understanding when fish actually captured their prey. We can try to reconstruct the timing of feeding events using complicated models, but there is substantial uncertainty involved.

In this paper, I compared real-time observations of salmon feeding behavior collected by snorkeling to a time series of stomach fullness collected by sampling diets. Using real-time observations of behavior, it was clear that salmon fed most often in the morning, declining through late afternoon, and that salmon fed throughout the day. Examining diets alone, we could only tell that salmon fed during the morning (their stomachs were near empty at dawn), and it was unclear that they had slowed feeding during the day or fed throughout it.

This means (1) juvenile salmon feed often, which makes sense because they must grow quickly to overcome the jaw sizes of predators, fuel migration, and survive their first winter when prey is scarce, so we should ensure that we haven’t compromised the prey availability of their nearshore habitats (e.g., by building along shorelines), and (2) we probably have a vague understanding of when many fish feed because we’ve only examined their diets rather than real-time behavior.

Relevant publication

Munsch, SH, Cordell, JR, & Toft, JD (2017). A Quantitative Chronology of Diurnal Feeding in Juvenile Pacific Salmon. Transactions of the American Fisheries Society, 146, 222-229.

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