Feature Conference Presenter: Craig Dahlgren

Bonefish pond












Craig will be speaking on the efforts to improve the Bonefish Pond
area on New Providence – a summary of this project here. Craig may
know more about Bahamian marine ecosystems than any scientist
working in the country.  You can find many of his papers in the archive.
One of my favorites was his analysis of habitat shifts in Nassau Grouper.

In this paper, he explores what might be the most fundamental trade-off
for all animals – finding food without being eaten yourself.  This trade-off
could explain the distinct “ontogenetic” (i.e., as an animal grows) habitat
shifts of Nassau Grouper.  The smallest grouper are found primarily in algae
clumps (often the red algae Laurencia), larger ones shift to more complex
structure such as rocky outcroppings or patch reefs, and adults on
the forereef.  Craig demonstrated that the trade-off between eating and
being eaten explained these shifts.  The smallest grouper have a very low
risk of predation in algal clumps, but once they reach a certain size, it
is more beneficial to shift to non-algal habitats where they can maximize
growth rate.  The paper summary after the jump.

Distribution of mobile animals may reflect decisions on how to balance conflicting demands associated with foraging and avoiding predators. A simple optimality model predicts that mobile animals should respond to changes in mortality risk (m) and growth rate (g) by shifting habitats in a way that maximizes net benefits. In this study, field caging and tethering experiments quantified habitat-specific growth rates and mortality risk, respectively, for three different sizes of a coral reef fish, Nassau grouper (Epinephelus striatus), during its juvenile tenure in off-reef nursery habitats. These sizes bracketed the size at which this species undergoes an ontogenetic habitat shift from the interstices of macroalgal clumps (‘‘algal habitat’’) to areas outside, or adjacent to, macroalgae and other physically complex microhabitats (‘‘postalgal habitats’’). Experimental results were used in a cost–benefit analysis to test the following alternative (but not mutually exclusive) hypotheses: (1) juvenile grouper shift habitats in a way that maximizes growth rates (g); (2) juveniles shift habitats in a way that minimizes mortality (predation) risk (m); and (3) if trade-offs exist between maximizing growth rate and minimizing mortality risk, juveniles
shift habitats in a way that minimizes the ratio of mortality risk to growth rate (m/g). Results suggested that small fish face a trade-off between living in the relatively safe algal habitat and achieving high growth rates in postalgal habitats. The value of m/g was significantly lower in the algal than postalgal habitats for small fish, which typically reside in the algal habitat, and significantly lower in postalgal habitats for medium and large fish, which typically reside in postalgal habitats. Thus, habitat use by juvenile Nassau grouper was consistent with the ‘‘minimize m/g hypothesis.’’ These results highlight how behavioral responses to ecological processes, such as changing predation risk with body size, determine distribution patterns of mobile animals.

A juvenile grouper on one of our cement block reefs. This fish likely just has left the protection of algal habitat for this reef where food is more abundant.

A juvenile grouper on one of our cement block reefs. This fish likely just has left the protection of algal habitat for this reef where food is more abundant.

By | 2017-12-01T14:02:55-05:00 February 22nd, 2014|Categories: Fish, Mangroves and Creeks, marine protected areas, Restoration|0 Comments

About the Author:

Craig Layman
My lab’s interdisciplinary pursuits provide for a multi-faceted understanding of environmental change in the coastal realm. We are ecologists, asking questions that span population, community, ecosystem and evolutionary sub-disciplines. We often use a food web based perspective, exploring top-down (e.g., predation) and bottom-up (e.g., nutrient excretion) mechanisms by which animals affect ecosystem processes. All of our efforts are framed within a broader outreach framework, directly integrating science and education, using approaches such as this website.

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