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A Rainbow Parrotfish (Scarus guacamaia) Image By: Michelle G. Pasquin. Copyright 1998. Bermuda Aquarium Museum & Zoo, Bermuda.
 

According to optimal foraging theory, an organism will forage in a manner that optimizes fitness (Pyke, 1984). The results from this study indicate that the strategy used by the Rainbow Parrotfish (Scarus guacamaia) is one in which it takes short bites at a high frequency, moving among sites between each grazing period. More specifically, the optimal foraging strategy is one in which the number of bites is clumped around six, and the length of each bite at each site is generally short, at one or two seconds each. The Rainbow parrotfish move between foraging sites after each several second period of grazing. This is presumably done to obtain the “best” algae at each site and move on, rather than graze away all the algae in one area.

In other systems the herbivorous foraging strategy is governed by the time required to take each bite and the time it takes to process each bite (Illius et al 2002). Here, the parrotfish is taking smaller bites and thus taking less time to process them. In addition, Illius and coworkers (2002) suggest that increasing bite rate compensates for declining bite mass in other herbivores. The bite rate in this study is clumped around six bites per minute, tending towards a shorter grazing length (mean of 2.3 seconds). So the Parrotfish’s higher bite rate is compensating for its shorter bite length.

            One possible explanation for this behavior is the limited amount of algal growth on the reef. As there are many species of herbivorous fishes in the coral reef community the growth of the algae is limited by herbivory (Deloach, 1999). Thus, in any given area there is not an abundance of algal growth. Therefore it is more beneficial for the parrotfish to spend less time in a greater amount of grazing spots than a lot of time in one specific area. Indeed it has been shown that herbivores in Caribbean reefs compete for food and space (Bruggemann et. al., 1994c). This was demonstrated when the removal of Diadema antillarum resulted in a higher density of herbivorous fishes present (as cited in Bruggemann et al, 1994c). Thus, the Rainbow Parrotfish is under pressure to get the most nutritional value out of each bite it takes and cannot afford to waste time foraging at inopportune sites.

            It is also possible that the high frequency of bites per minute at multiple sites is representative of a substrate preference in the Rainbow Parrotfish. Bruggemann and colleagues (1994c) showed that amongst two species of Parrotfish in competition for foraging resources a substrate preference was present, such that one species preferred flat surfaces while the other preferred concave grazing surfaces. This observable fact was then shown to be related to grazing mode in each species. In Scarus guacamaia a similar event may be occurring, such that the fish moves frequently between grazing site in order to find sites with substrates optimally suited for its mode of feeding. In addition substrate density can also affect the amount of food obtained per bite: low density substrates enable deeper bites with higher yields than high density substrates (Bruggemann et al, 1994a). Thus, the Rainbow Parrotfish would forage more effectively on low density substrates and may have been actively searching out areas where a short bite would produce a high yield, given that the majority of the bites taken were short.

            Alternatively, the foraging strategy employed by the Rainbow Parrotfish could be related to food preference. Bruggemann et al (1994a) have shown that in the species of parrotfish, Sparisoma viride, there is a preference for food containing the highest levels of protein or energy. Thus, it is feasible that the Rainbow Parrotfish also has a preference for certain food items. In using a foraging strategy that is intended to maximize energy intake per foraging effort, it is probable that high protein or high energy sources are preferable to low energy sources. Foray size is a good indicator of food preference (Bruggemann et al, 1994a). Thus, the Rainbow parrotfish will take longer forays on food and substrate types that enable the highest yield per bite (in both energy and biomass). During the periods of foraging observed in this study, the Rainbow Parrotfish took more shorter bites than longer ones (Figure 2). Thus, it is possible that there was not a wide availability of the food source which the Rainbow Parrotfish most preferred. Therefore it chose to take a high frequency of short forays as opposed to a low frequency of longer forays. However, it can be postulated that when S. guacamaia foraged for a longer period of time a more preferable food source was present. Thus, the observed foraging behavior may have been governed by the availability of food type.

            It has been demonstrated that the Parrotfish species Sparisoma viride varies its feeding rate with the time of day (Bruggemann et al, 1994b). My study was limited by time available to conduct observations and thus there were only two observational periods in which data collection was preformed: once at 10:00 am and again at 4:00 pm. Therefore an observational bias in recording of foraging behavior may have occurred due to variation in foraging with time of day. In order to correct for this, more studies should be conducted in which Parrotfish are observed for all hours of the day for a period of several days. This would allow for differences in foraging strategy with time of day to be noted and accounted for. In addition, a larger amount of data collected would have been useful in providing more significant results for this study, but was not attained due to time constraints.

            Feeding rate (the number of bites per hour) has been shown to vary with size in the species of Parrotfish, Sparisoma virde. This variation may result from differences in bite rate or from time spent on swimming or social interactions (Bruggemann et al, 1994b). Smaller fish have been shown to spend less time on other activities and more time foraging than larger individuals (Bruggemann et al, 1994b). This study did not take size of the fish into account when observing foraging behavior and thus did not observe differences between individuals of varying sizes. However, the data could have been influenced if individuals of one size (most likely small) were observed with greater frequency than individuals of another size class. If small individuals were observed more often foraging rates would appear higher than they would be for the population as a whole. This bias could be corrected for by performing another study in which size classes were observed in addition to foraging strategy.

Herbivory by reef fishes has been shown to dramatically affect the community structure of the reef (Lewis, 1986; and Carpenter, 1986). Herbivory keeps the community dynamics in check by preventing algae from overgrowing the reef. Spatial variation in the intensity of herbivory is of fundamental importance in determining patterns of benthic community structure on tropical reefs (Lewis, 1986). In addition, plant-herbivore interactions result in higher overall ecosystem productivity and facilitate the flow of energy and materials from highly productive algal turf to higher levels in the reef trophic web (Carpenter, 1986). Thus, the role of foraging behavior in Scarus guacamaia is of key importance in determining the structure of the reef community. In the future, studies can be conducted to investigate herbivory by S. guacamaia and exactly the role in plays in the coral reef ecosystem.