The Causes and Consequences of Movement of Temperate Reef Fishes

For my dissertation, I investigated the movement patterns, their causes and consequences, of adult reef fishes on shallow temperate rocky reefs in the eastern Pacific. Most reef fishes exhibit two fundamentally different forms of movement throughout their lifetime: first the dispersal of larvae and later the movement of adults in the benthic environment. Because of the great potential for larval dispersal that characterizes the majority of marine species, this life history pattern has lead to a paradigm of open populations (i.e. reproductive output and recruitment of juveniles to a local population are decoupled) in reef fishes and a focus on larval dispersal and recruitment in reef fish ecology.   

Fish species tracked (From Left to Right), Kelp Greenling female (left) and male (right) (Hexagrammos decagrammus), Blue Rockfish (Sebastes mystinus), Kelp Rockfish (Sebastes atrovirens)

In temperate and tropical reef ecosystems, movement of adult reef fish has received much less attention by ecologists than the dispersal of larvae. Nevertheless, the importance of movement patterns and distances of adults to the population dynamics of reef fishes has recently been recognized. There are three primary reasons for the increased interest in the movement of the adult life stage of reef fishes. 

  • Larval dispersal of reef fishes might be more limited than previously thought.
    This means that at reduced scales of larval connectivity the movement of adult individuals might have important population level consequences (i.e. demographic and genetic connectivity). 

  • Species movement is a driver of ecosystem function and resilience.
    Movement is a fundamental determinant of the strength and diversity of interactions that individuals and species have in ecosystems, including ecosystem connectivity. 

  • Spatially explicit management approaches including marine protected areas (MPAs) have lead to an increased interest in movement of individuals on smaller spatial scales (i.e. 100’s of meters) and shorter temporal scales (i.e. days) than previously considered in fisheries management.  

Methods

VRAP buoy array.  The three hydrophone buoys can locate the position of a fish by the time differences it takes the acoustic signal that is send by the tag to travel  to the three buoys.  Locations are radioed to a base station where the data is displayed in real time.  This system tracks individuals at high spatial resolution (meters) for long time periods (years).  The array is shown over a topographic map of the seafloor.

I used high resolution acoustic telemetry (VRAP system, by VEMCO Ltd.,) to track the movement of individual fish from three species: kelp greenling (Hexagrammos decagrammus), kelp rockfish (Sebastes atrovirens) and blue rockfish (Sebastes mystinus).  Fish were captured and acoustic tags were surgically implanted.  Using this approach, I experimentally investigated the effect of population density on the movement of kelp greenling and studied the effect of habitat variability on the movement patterns of kelp rockfish and kelp greenling.  Further, I analyzed published movement distances of temperate reef fishes along the North American west coast.

Analysis and results

This analysis of published movement studies showed that movement distances of most species that have been studied are characterized by a positively skewed frequency distribution and discrete home ranges rather than by unbounded, diffusive movement rates. For 80% of the species the 75th percentile movement distance was less than three kilometers.

The acoustic movement studies clearly showed that kelp rockfish and kelp greenling have small home ranges on scales of several hundred square meters. Blue rockfish did not show site fidelity at the spatial scale of this study and seem to move over greater distances.

The experimental manipulation of population density demonstrated a positive relationship of population density and the home range size of male kelp greenling. I found a 55% reduction in home range size of male kelp greenling after their density was reduced by half.

Kernel density estimates of home ranges.  The figure is showing the overlapping home ranges of two individuals.  The height of the curves indicated the probability of an individuals being at a certain location.  The solid lines outline the 50th and 95th percentile kernel density.  The 95th percentile outlines the home ranges of these individuals (i.e. the area in which the fish is found 95 % of the time).   

Based on the frequency of association of individuals with certain habitat features, red algae and giant kelp (Macrocystis pyrifera) were identified as important habitat variables in kelp greenling and kelp rockfish home ranges, respectively. The abundance of these algae affected the home range size of these species. Male kelp greenling home range size correlated inversely with the density of red algae, whereas kelp rockfish home range size was positively related to changes in kelp abundance. These contrasting responses are consistent with the species use of the respective algae as foraging habitat (red algae) or shelter (giant kelp). 

My dissertation combined analytical, experimental and monitoring approaches to investigate the movement of temperate reef fishes. Limited and discrete movement ranges reinforce the spatial heterogeneity of reef fish populations created by larval settlement and post-settlement mortality, including fishing. Knowledge of how movement is influenced by local population density is critical to understanding how movement contributes to the regulation of local and regional populations. Effects of habitat variability on the movement of species are critical to their population ecology, species interactions and community dynamics in a changing environment.

Divers capture fish using hook and line fishing
while scuba diving. Acoustic tags a surgically implanted
on the boat and fish are released at the capture site.
 The tags will send acoustic signal every few minutes
that can be tracked by the hydrophone array. 
 

 Understanding the causes and ecological consequences of movement is also important to the management and conservation of these economically and ecologically important species. Specifically, ecosystem based management that focuses on the ecological functions of species within communities needs to manage reef fishes on spatial scales over which the ecological roles of local populations are realized. Therefore, the movement of individuals, both larval and adult, and its dependency on ecological covariates determines the spatial scales of these management approaches. Further, the design and effectiveness of MPAs depend on the spatial scales of the movement of species targeted for protection and the way in which their movement changes after the protection of communities. Investigating movement and its response to changes in density and habitat is critical to informing these management approaches.

For more information on this project contact:

Jan Freiwald, PhD
Phone: (831) 459-5783
Fax: (831) 459-3383
freiwald@biology.ucsc.edu

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