van Jules Rooij

The social and mating system of the herbivorous reef fish Sparisoma viride: One-male versus multi-male groups

SynopsisWe present a detailed description of the social and mating system of the protogynous reef herbivoreSparisoma viride at the fringing reef of Bonaire (Netherlands Antilles). Initial phase (IP) fish and terminal phase (TP) males occur either in one- or in multi-male groups, which are compared quantitatively with respect to the use of space, size composition, social interactions and sexual activity. One-male groups consist of one TP male plus 1–14 IP females, whereas in multi-male groups up to 14 TP males and about twice as many IP fish share a common home range. The two social units further show marked differences in vertical distribution (one-male groups are restricted to depths between 3 and 22 m, multi-male groups mainly residing < 3 m), size composition (a larger proportion of small adults live in multi-male groups), size and stability of the range (larger in one-male groups) and sexual activity (daily spawning of one-male group members inside their normal home range; no activity in multi-male groups on the shallow reef). Sexual activity occurred daily, throughout the year, with no evidence for tidal tracking or major seasonal or lunar patterns. The one-male groups constitute less than 20% of the adult stock but control up to 77% of the inhabited reef. As a result they have access to higher-yield food patches and to suitable spawning sites inside their home range. Although members of both units appear to defend their common home range against intruding conspecifics, the degree of territoriality is clearly higher in one-male groups. We discuss the relative importance of food, shelter, mates and mating sites as defended resources and some life history implications. The complex social and mating system ofS. viride shows much resemblance to that of another Caribbean scarid,Scarus iserti. This complexity seems to reflect the capacity of individuals to flexibly adapt their feeding, mating and life history styles to an unpredictable environment. We propose thatS. viride is a good study animal to test adaptive explanations for its territorial organization and complex life history patterns.

Resource and habitat sharing by the stoplight parrotfish, Sparisoma viride, a Caribbean reef herbivore

SynopsisWe compare the vertical distribution, substrate preferences, grazing behaviour and social interactions of the stoplight parrotfish,Sparisoma viride, with that of other scarids at a fringing reef off Bonaire (Netherlands Antilles). Earlier reports thatS. viride only displays territorial behaviour against conspecifics are confirmed by the non-aggressive nature of this parrotfish and the low time expenditure on interactions with other herbivores. The vertical distribution ofS. viride is largely identical to that of several other scarids, acanthurids and pomacentrids, whose ranges may completely coincide withS. viride territories. Comparison of the substrate use of the five most common scarids inside a singleS. viride territory yields no evidence of food partitioning. We suggest that the lack of interspecific territorial behaviour inS. viride is explained by the inability to economically defend a territory against all potential food competitors. Factors that may favour territory sharing between herbivores are fine-scale resource partitioning and shared defence, both of which would reduce the costs of territorial life. However, more detailed investigation of herbivore food selection is required before definite conclusions can be drawn.

Local variability in population structure and density of the protogynous reef herbivore Sparisoma viride

SynopsisWe compare the (relative) abundance of life phases [juveniles (JU), initial phase (IP) and terminal phase (TP) fish], social categories (territorial and group adults), and fish following alternative mating styles, in three local populations of the protogynous reef herbivore,Sparisoma viride, on the fringing reef of Bonaire (Netherlands Antilles). In order to determine the adaptive significance of variations in social organization, they are related to the density of conspecifics and other herbivores and to the availability of food, shelter and mating sites. The most striking difference is the high abundance of JU and group fish at one location (Playa Frans) and the total absence of group fish at another (Red Slave). These differences are coherent with a gradient in population density, total herbivore density, scarid grazing pressure, and reproductive output, all of which are highest at Playa Frans and lowest at Red Slave. Exposure to waves and currents shows an inverse trend. The differences in the relative abundance of territorial fish can be explained by the concept of economic defendability, which is reduced at higher population density. In a life history context, small TP group males represent ‘bachelors’ that sacrifice current reproduction for better future prospects. As predicted by life history theory, early sex change is promoted at sites where the future rewards are higher (higher spawning rates of large TP males) and where the costs incurred during the bachelor phase are reduced (more spawning opportunities for group TP males). At Red Slave an alternative male mating style (‘streaking’) appears to be promoted by the lack of a refuge for group TP males and by a dense gorgonian canopy, allowing IP males to reside inside territories. We conclude that most observed differences in population structure can be considered adaptive in an ecological and in a life history context. Population density is a major factor in both contexts. Analysis of the variability in adult density in relation to JU density and the availability of food and shelter indicates that theS. viride populations at Bonaire are not totally controlled by stochastic processes. Considering the small spatial scale and the high dispersal of the planktonic embryos and larvae, the observed variability in behavioural and life history traits ofS. viride points to a high degree of phenotypic plasticity.

A simple field method for stereo-photographic length measurement of free-swimming fish: merits and constraints

Abstract A simple field method to correct for bias in stereo-photographic underwater measurements is presented and its usefulness to improve the accuracy of length estimates of free-swimming fish is tested. The calibration is based on the inclusion of stereo exposures of a slate of known dimensions at the beginning and end of each measuring session, and it allows correction for non-parallel optical axes, spherical aberration of the lenses, and for other sources of bias that may vary between sessions. A precision of ±3% for replicate length measurements of a 30 cm slate is obtained at distances between 0.7 and 2.0 m. This compares well with reported values that are obtained with more sophisticated (laboratory) methods. However, application of this calibration does not significantly improve the accuracy of stereo measurements of the length of free-swimming fish (±2.5 cm or 7–11% of actual fish length). The limitations are caused by difficulties in recognizing identical extreme points at the body of fish that are photographed under field conditions. By averaging three or more replicate measurements the accuracy is improved to ±1 cm (2.7–4.5%). We conclude that effort should be aimed at increasing sample size, rather than at improving equipment and correction procedures, when measuring free-swimming fish in their natural habitat.

Analysis and Comparison of Fish Growth from Small Samples of Length-at-Age Data: Detection of Sexual Dimorphism in Eurasian Perch as an Example

Abstract A relatively simple approach is presented for statistical analysis and comparison of fish growth patterns inferred from size-at-age data. It can be used for any growth model and small sample sizes. Bootstrapping is used to generate confidence regions for the model parameters and for size and growth rate at age. Significance of differences between growth patterns is tested with a likelihood ratio test, the validity of which was confirmed by Monte Carlo simulations. As an illustration of the applicability of this approach, we analyzed a set of length-at-age data on female (N = 57) and male (N = 27) Eurasian perch Perca fluviatilis from a shallow, eutrophic Dutch lake, Tjeukemeer. We used the von Bertalanffy growth curve to model length at age for both sexes separately. This analysis showed a significant difference in length between males and females between age 2 and age 6, with females being larger. At older ages the confidence regions became too wide to detect sexual dimorphism. The applicability...

Ontogenetic, social, spatial and seasonal variations in condition of the reef herbivore Sparisoma viride

We studied the condition of stoplight parrot-fish, Sparisoma viride, collected from the fringing reef of Bonaire (Netherlands Antilles) between March 1987 and October 1991. To this end, we compared length-weight relationships using analysis of covariance. The condition of different life phases [juveniles, initial phase (IP) and terminal phase (TP) adults] and social categories (territorial and group adults) is compared in different seasons, taking into account spatial variability along a depth gradient. Variations in condition are related to differences in behaviour, the use of space and the distribution of food. The average length-weight relationship of fish with empty guits is described by the equation: WEmpty=9.115x10-6xFL3.140 (n=386, R2=99.82%, W in g, FL in mm). Territorial TP males were found to be in poorest condition, whereas (sexually inactive) group TP showed the best and IP fish an intermediate condition. These differences are ascribed to varying investments in territory defence and reproduction, which affect condition more than the vertical distribution of food. All categories were in minimum condition between May and July, the season with longest day lengths and increasing water temperatures. This seasonal effect cannot be ascribed to increased reproductive effort and suggests that the higher energetic demands due to the longer active period of fish are not completely met by increased food uptake. It is inferred that food may be a limiting factor at times.