Incorporating the Functional Response of Rock Crabs into Ecosystem Management in California

— Written by Gabbie Baillargeon

After several weeks of spending quality Friday nights with my crabs recording behavioral observations during feeding trials, I finally had enough data to begin to piece together the larger puzzle.  The goal of this study was to measure rock crab’s functional response, or the per capita foraging rate of a predator across varying mussel densities, in order to gain a deeper understanding of the predator-prey relationship between rock crabs and California mussels in the nearshore California ecosystem.  The premise of this research is grounded in the knowledge that rock crab density varies naturally and aggression is common between individuals, and thus harvesting this species could have consequences for species interactions and community structure.  The results of this study show that the functional response of rock crabs at all three predator densities all fit a Hollings Type II functional response curve when plotted.  This means that predation rate increases with prey density until the organism becomes full, then the predation rate plateaus.


Predator density of 2 crabs, feeding on 10 mussels

Comparing the predation rate of rock crabs on mussels, the results show conclusively that the predation rate was lower at all prey densities for the three-predator density in comparison to the two-predator density treatment.  This indicates that predator interference, most likely in the form of rock crabs fighting with each other as more crabs are present, decreases the number of mussels consumed per predator. However, it should be noted that the number of mussels consumed per tank did increase in proportion to predator density.  This means that per capita mussel mortality, or the chance of a mussel being consumed, does increase with the addition of more predators.  Breaking mussel consumption down on a per predator basis results in lower number of mussels being consumed per predator at higher predator density treatments due to dividing the number of mussels consumed by larger numbers of predators.

Behavioral studies show that number of aggression events, quantified by fights or aggressive posturing, were highest at the three-predator treatment.  However, there was no significant difference between average number of aggression events between the two and three predator treatments.  The size selection of mussels across all treatments was also measured in this study.  It was found that the average mussel consumed was 67 mm and that there was no significant difference between the average mussel size selected across predator treatment groups.

The main conclusion about the functional response of rock crabs is that they fit a Type II functional response and the shape of the curve is controlled by predator density, making it a predator dependent response. The first implication of this research is that rock crabs are clearly able to exert pressure on mussel populations, influencing their population dynamics.  Another important implication of this research is its application to fisheries management.  Pacific Rock Crabs and California Mussels are both harvested species and have strong trophic linkages to other organisms in the food web.  As fishing activity alters rock crab abundance, the biological interactions between rock crabs changes and in turn the degree of mussel consumption and availability of mussels for other species and harvesting fluctuates correspondingly.  Ecosystem-based management and co-management of species calls for better data that aims to understand key drivers of predator-prey relationships, such as the functional response.  The functional response is an important component of predator-prey relationships, and in the case of the rock crab, it presents strong evidence that rock crab stock assessments should be conducted in order to better assess the health of the population.  My research on these feisty crabs clarified one small piece of the puzzle in terms of learning how to incorporate knowledge of the nature of predator-prey relationships through studying the functional response in order to successfully implement ecosystem based management plans for fisheries.

Crabs Gone Wild: A Look at the Mysterious Foraging Behavior of Rock Crabs

— Written by Gabbie Baillargeon


Two female brown rock crabs (Cancer antennarius)

Any research involving the study of living animals is simultaneously exciting and frustrating due to the fact that lab experimentation involves manipulation of individuals who have a brain of their own and don’t always want to cooperate nicely.  Although the general goal of my research project is to study the foraging behavior of brown rock crabs on California mussels, the specifics within addressing this research query have shifted in response to my preliminary observations of crab behavior.  Through this research project, I have come to find that doing science is a much more elaborate, drawn out, and unpredictable process of discovery than the simple flowchart of the scientific process in my high school biology book.

In order to start experimenting on the rock crabs, all of the crabs must undergo a set acclimation phase where they are allowed to get settled into their new, luxury tank habitat.  Following acclimation, the plan was to start all of the crabs in individual trials where both crab density, number of crabs per tank, and prey density, number of mussels per tank, would be systematically manipulated.  Simply, this experiment involves changing two variables: food availability and crab predator abundance, with the goal being to measure how the crabs respond to these changes by counting the number of mussels consumed and documenting behavior during feeding. All was going according to


A gravid female yellow rock crab

plan for the first few weeks as crabs came into the lab they were placed in tanks, fed, and played nice with one another. However, due to little previous research published or documented observations available for reference, I had the privilege of witnessing first hand a wide array of surprising crab behaviors before the experiment even began.  For those of you who, like myself, were blissfully unaware of rock crabs’ night time job as a ninja, I am here to tell you that they are exceptionally stealthy little beasts who are capable of destroying every plastic tank barrier I repair. Without the tank barrier in place properly, crabs could move freely between the tanks which makes it impossible to accurately measure which crabs consumed which mussels – a vital aspect of data collection.  Along the way, a few crabs were lost in the battle for scientific understanding as some crab fights would end in death for the loser.  There was even an instance of crab suicide, where the crab simply found a way to escape its tank and met its unfortunate end drowning in the open air.   Additionally, many of the female crabs collected started to become gravid, or possessing eggs, during the acclimation period. To avoid any bias in the study, all females who were not gravid were going to be used in the experiment.


Measuring and labelling a new batch of rock crabs to prepare for running foraging trials

A series of small pre-experiment foraging trials were conducted to get a sense of how the crabs interact with each other, how many mussels they consume, and how they utilize tank space.  The most surprising find of this small study was that groups of female crabs consumed many more mussels than groups of male crabs, indicating that there is a difference in their foraging behavior.  Given this new information, along with the predicament of females become pregnant, the experiment was reconsidered to answer a slightly different question within the same frame of investigating the interface of rock crab foraging and human impact on their populations.  My experimental design shifted to reflect this new path of investigation, as the study now tested only males. To accommodate a shortened timeline and restricted tank availability, the number of prey and predator densities was altered as well.   All in all, the crabs provide a good laugh, a frustrated yell, and sometimes a proud smile along the journey of designing and running foraging trials to better understand their feeding preferences and behaviors.



Studying Santa Barbara Fisheries: Spotlight on Rock Crabs

— Written by Gabbie Baillargeon


Walking along Stern’s Wharf, accompanied by the bustling crowd of downtown Santa Barbara tourists and locals alike, signs proudly boasting the sale of delicious rock crabs seem to line the boardwalk.  Similar to their more famous and tasty cousin, the Dungeness crab, rock crabs are a group of crab that comprise a productive and lucrative fishery along the coast of Southern California and the Northwestern Pacific states.  Three distinct species of rock crab are found off the shores of California: Yellow (Cancer anthonyi), Red (Cancer producutus), and Brown (Cancer antennarius) rock crabs.  All three species are available to purchase as seafood, though red rock crabs have the highest demand due to their larger size and sweeter meat.  For the amount of recreational and commercial fishing that surrounds Rock Crabs, shockingly little research has been dedicated to studying them and even less information is available to shed light on how these crabs operate in the unique environmental conditions of Santa Barbara.


Two yellow rock cabs inside a cinderblock, a favorite resting spot.

My research project investigates how changing fishing pressures on rock crab populations alters their foraging behavior.  In order to mimic fishery pressure, different densities of rock crabs will be put in controlled environments and fed different densities of California mussels (Mytilus californianus) to measure their predation rates in each case. The goal of this project is to determine if changing the number of rock crab competitors of a single species will affect how they forage.  In an effort to better understand how fishing plays a role in shaping the nearshore marine environment, the potential consequences of varied foraging behavior in response to population shifts on the larger ecosystem will be analyzed.


A Brown rock crab being measured and tagged after collection from Stern’s Wharf.

In the early stages of the project, plenty of challenges quickly multiplied when working with the rock crabs or “little monsters” as we fondly like to think of them.  The rock crabs have earned this nickname as they seem to have a bit of a angst-ridden destructive streak in them, especially when it comes to following some basic tank rules.  After only 24 hours, the crabs had deftly removed their identifying leg bands – I’m sure they planned this operation carefully and helped each other out.  Then, instead of being happy with their buffet of mussels and clean tank to play in, some crabs decided that they simply must explore what was on the other side of the tank barrier and crawl into their neighbor’s tank.  The mussels are bluer on the other side, right? Lastly, although the names red, yellow, and brown rock crab seem to indicate that it would be simple to distinguish between the three species, I painfully came to find out that could not be farther from the truth.  Despite the roadblocks, learning to care for the crabs and become excited over little things such as a crab finally deciding to eat mussels, has all been a part of the learning curve and given me invaluable skills of patience, problem-solving, and a lightning-fast reaction to a feisty crab claw.