Linda Robb

TEMPORAL PATTERNS OF MOVEMENT IN JUVENILE FLATFISHES AND THEIR PREDATORS - UNDERWATER TELEVISION OBSERVATIONS

Underwater television cameras were used to observe the movements of bottom-living animals in the intertidal and shallow subtidal zones of a sandy shore over sixteen 24-h periods in the summers of (1991) and (1992). Juvenile (O-group) flatfishes, predominantly Pleuronectes platessa L. with some Limanda limanda (L.), were only seen moving on the bottom by day, with most observed at high water and just before sunset. Their movements were directed offshore in the morning and onshore in the afternoon. They also tended to move onshore with the flood tide and offshore with the ebb. These fishes may use midwater swimming for intertidal migration by night because none was seen moving on the bottom at night. Potential predators of O-group flatfishes, cod Gadus morhua L., crab Carcinus maenas (L.) and the brown shrimp Crangon crangon (L.), were most often seen moving on the bottom at night. Crangon, Carcinus and I-group flatfishes were seen in similar numbers in the subtidal and intertidal zones, while O-group flat-fishes, cod and hermit crabs Pagurus bernhardus (L.) were seen much more frequently under the subtidal camera.

The Behavioural Basis of Predator-Prey Size Relationships Between Shrimp ( Crangon Crangon ) and Juvenile Plaice ( Pleuronectes Platessa )

The shrimp, Crangon crangon (L.) (Crustacea: Crangonidae), is a significant predator of the smallest sizes of plaice, Pleuronectes platessa L. (Teleostei: Pleuronectidae), during and immediately after the fish settle on sandy beaches when predation rate is strongly dependent on the size of both the predator and the prey. Laboratory experiments showed that this size-dependency is caused principally by the superior escape capabilities of larger fish once captured rather than differences in the ability of different sizes of shrimps to capture their prey. Fish that escape after capture are often wounded and some of these wounds may subsequently be fatal. Many shrimps capture and eat fish that are larger than their stomach volume resulting in long handling times and low prey profitabilities. For all sizes of shrimps used (36–65 mm total length) prey profitability (mg prey ingested min −1 ) increases with decreasing fish length.

Spatial scales of variance in abundance of intertidal species: effects of region, dispersal mode, and trophic level.

Determination of the pattern of variation in population abundance among spatial scales offers much insight into the potential regulating factors. Here we offer a method of quantifying spatial variance on a range of scales derived by sampling of irregularly spaced sites along complex coastlines. We use it to determine whether the nature of spatial variance depends on the trophic level or the mode of dispersal of the species involved and the role of the complexity of the underlying habitat. A least-cost distance model was used to determine distances by sea between all pairs of sites. Ordination of this distance matrix using multidimensional scaling allowed estimation of variance components with hierarchical ANOVA at nested spatial scales using spatial windows. By repeatedly moving these spatial windows and using a second set of spatial scales, average variance scale functions were derived for 50+ species in the UK rocky intertidal. Variance spectra for most species were well described by the inverse power law (1/fbeta) for noise spectra, with values for the exponent ranging from 0 to 1.1. At higher trophic levels (herbivores and carnivores), those species with planktonic dispersal had significantly higher beta values, indicating greater large- than small-scale variability, as did those on simpler coastlines (southwestern England and Wales vs. western Scotland). Average abundance and proportional incidence of species had the strongest influence on p values, with those of intermediate abundance and incidence having much greater large-scale variance (beta approximately 0.5) than rare or ubiquitous species (beta approximately 0).

Topography as a determinant of search paths of fishes and mobile macrocrustacea on the sediment surface

Abstract Reworking of the sediment surface by the movement and feeding activity of mobile epifauna is likely to affect rates of solute flux across the sediment–water interface, with consequences for the geochemistry of deeper sediment layers. Mobile epifauna are also important scavengers and predators that control the community of sediment infauna and affect the distribution and abundance of other surface-living species. Activity of subtidal epifauna in a sea loch was monitored by video in order to test the hypothesis that patchiness of visits by epifauna to small areas of sediment surface is generated by direct behavioural responses to surface features, such as mounds and holes built by large infauna, and as an alternative hypothesis that patchiness is generated by the properties of the search paths as measured by rates of turning and step lengths. Trajectories of gobies (Lesueurigobius friesii, Pomatoschistus minutus), shrimps (Crangon allmani), and Nephrops norvegicus were digitized in 12 half-metre square quadrats over 24 h. Analysis of frequency of visits to small areas within each quadrat quantified the patchiness of impacts of each species. Trajectories tended to be area concentrated and in many cases more than would be expected on the basis of search path properties alone. Spatial patterns of frequencies of visits showed that movements of gobies L. friesii, P. minutus and Nephrops were strongly influenced by proximity to burrow openings, while shrimps (Crangon) were not affected. Observed patchiness in P. minutus was positively correlated with simulated patchiness, suggesting that properties of search paths may change in response to food and consequently affect patchiness. Crangon allmani often followed furrows, also increasing patchiness. The effect of the characteristics of search paths on patchiness of visits was small compared to the direct response to topographical features.