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December 29, 2009

DNR biologists utilize video to assess coastal fish populations

Assessing the abundance and behavior of coastal fish populations presents unique challenges to marine biologists. In an attempt to address this, a collaborative effort to monitor fish recruitment to and utilization of a small artificial reef using a remote video camera system was initiated in 1999 between researchers from the S.C. Department of Natural Resources (DNR) and the Skidaway Institute of Oceanography (SKIO). 

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The reef consisted of fabricated concrete units similar to those designed by and used by the DNR Artificial Reef Program.  This reef was situated at an existing research site on the middle continental shelf off the northern Georgia coast in order to take advantage of intensive environmental monitoring of that area by SKIO researchers.  The camera system collected short duration (10-second) video clips once per hour during daylight and periodically transmitted the images to shore via microwave technology.  The location of the reef was not disclosed to the general public in order to study the animals utilizing the reef under conditions that were as natural as possible in an un-fished condition.
           
During the eight-year study, more than 77,000 individual video clips were collected and manually reviewed in the laboratory to identify and document the relative abundance of the observed animals. The study was funded by the Office of Naval Research and DNR Saltwater Recreation Funds.  A final report was prepared for the State Recreational Fisheries Advisory Committee (SRFAC) and is available online (PDF file). Find out more about the DNR Artificial Reef program.
           
Analysis of videos for 16 fish species or species groups revealed several noteworthy findings.  For example, Great barracuda, amberjacks and cobia were seen at the site for a longer portion of the year than was previously suspected, based on historical hook and line catches at similar water depths.  Occasional observation of cobia at the site during summer reaffirms that not all cobia migrate north to the Chesapeake Bay and may provide clues as to where cobia may be found after leaving Port Royal Sound in July.  Alternatively, fishes traditionally thought of as reef residents such as black sea bass, gray triggerfish, Atlantic spadefish and snappers and groupers were often not seen for days at a time, emphasizing that periods of movement occur among "resident" species.
           
Although fisheries video successfully documented when certain species or groups occurred, figuring out why they occurred when they did was less precise.  Predator-prey dynamics were implicated in the increased observation of small reef fishes (tomtate, vermilion snapper) and mid-water (scad, juvenile) fishes during the second half of the study concurrent with a decline in observation of known predators such as snapper, grouper, blue runner, amberjack and great barracuda. Statistical analysis of co-occurrence of any of the species or species groups examined only explained about one-fifth of the variation observed.  Seasonal and daily occurrence of species or species groups was also compared to water temperature, day length, salinity, wave height, time-of-day and tides; however, these environmental variables accounted for just one-fourth of the statistical variation observed.  Abundance of game fishes increased steadily during the first half of the study, peaking between 2004 and 2005, and then declined to levels comparable to those seen at the start of the study. 
           
Coincidentally, La Niña conditions and higher barometric pressures occurred during both the early and later years of the study whereas El Niño conditions and generally lower barometric pressures were associated with the middle years. Low frequencies of occurrence during summer and fall 2006 under El Niño conditions suggest that climate did not exclusively account for differences among the years.  Alternatively, food availability at the reef in later years may not have been sufficient to sustain and retain a large population of game fishes.    

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