Piscivore Ecology

School of greater amberjack. (Photo: Greg McFall)

The behavioral ecology of piscivores is being studied at Gray’s Reef to better understand the interactions that link predators and prey in the sanctuary. During a brief study last year Dr. Peter Auster from the University of Connecticut, along with colleagues and students, found that foraging by fish predators in the water column actually enhanced feeding opportunities for fish predators that hunt on or near reefs. For example, groups of greater amberjack, Spanish mackerel, and great barracuda (three fish species that feed on other fishes - piscivores) were observed using coordinated hunting behaviors, both within a species and among species, when preying upon mixed schools of juvenile tomtate (a small bottom fish) and round scad (locally known as “cigar minnows”). These hunting behaviors drove schools of small fish closer to reef habitats on the bottom and created feeding opportunities for bottom-dwelling scamp and gag grouper. The scientists reasoned that rather than competing for food resources, the behaviors of these fishes may result in more feeding opportunities for all.

The project this year expands on these initial observations and combines several methods of observing fish behavior and distribution to get an improved “picture” of the interactions between species.  The primary goal of this work is to determine the importance that these types of behavioral interactions have in local food webs.  Beyond basic ecological interests, the results of this work have important implications for managing marine protected areas.  Auster’s group, Dr. Laura Kracker from NOAA’s NCCOS in Charleston SC, and Greg McFall from GRNMS will collaborate at sea taking an interdisciplinary approach to the problem, using both direct underwater observation and hydro-acoustic techniques to observe fishes at multiple space and time scales. 

Echogram showing backscattering of the acoustic signal at 120kHz. The green lines indicate the sea floor, 2m from the bottom and 10m from the bottom. The color scale shows backscatter strength (Sv) in decibels. Mean Sv is measured in dBre1m-1 with greater backscatter strength (small schools) shown in blue and green. (Photo: Laura Kracker)

There are three basic objectives for this research cruise.  The first is to quantify the abundance of schooling prey and mid-water predators at mid-shelf reefs over space and time.  This involves conducting survey lines using a Biosonics scientific acoustic survey system – think fancy fish finder - to survey the water column and estimate the abundance and distribution of fish.  Active acoustic surveys use an echosounder to generate a sound that is sent into the water column. A transducer transmits the signal and then detects the reflected sound wave or echo. The strength of the returned echo tells you how big an object is (in fish it is the swimbladder that does most of the reflecting). Also, the length of time that it takes the sound wave to travel from the transducer, reflect off the object and return to the transducer will tell you how far down into the water column the target is.  The scientific echosounder produces an echogram similar to a fish finder. The echogram maps the entire water column, such that fish abundance can be determined.  Typically, acoustic surveys are run along transect lines at about five knots with a downward looking transducer mounted on the ship. Fish are counted and fish size is estimated based on the strength of the reflected acoustic signal. The output provides both a graphical map of fish distribution, as well as estimates of fish biomass.  There are several advantages to using this technology, such as sampling the water column ‘remotely’ without disturbing or extracting animals. Since acoustic technologies rely on the transmission of sound in water rather than light, it is a great advantage over using cameras in murky or very deep waters.  Information from acoustic surveys can be used to monitor changes over time, make spatial comparisons, or examine fish distribution in relation to bottom habitats.

Sonar image (1.8mHz) taken with a high resolution imaging unit May 2006 at Gray's Reef. Scale indicates distance (m) from lens extending out horizontally. (Photo: Laura Kracker)

The second objective is to quantify the rates of prey retreat to seafloor shelters.  This work will involve the use of a stationary high resolution sonar imaging unit deployed at the seafloor in a frame to look at specific sites over the time frame of hours.  The high resolution acoustic imaging system works on the same principle of sound in water as the Biosonics system described above but produces a more detailed image with a much higher resolution.  In these images, individual shapes and fish movement can be picked out. This device will be used to determine the rates at which predators “drive” prey towards the seafloor at specific locations and enhance feeding opportunities for piscivores living on and near the seafloor.

Finally, the third and last objective is to quantify the species composition, behavior, patterns of habitat use and rates of predation (attacks, captures) by mid-water and demersal piscivores.  This part of the project involves direct observations by divers using focal animal and scan survey procedures.  Underwater still photography and videography will be used to aid in data collection and document complex behaviors.  The use of digital imaging technologies greatly improves data collection when scientists can review images directly after dives to confirm species identifications, the elements of behavior, and other relevant observations.

The plan is to use all three approaches at three different study sites in and around Grays Reef NMS over the course of the cruise.  Replication of observations at each of the sites will allow scientists to understand the scale of variation both within and between sites.  The plan is to obtain observations during the day as well as at night with particular emphasis on dusk and dawn, the period where predator activity is most intense (we think).  Long days and night will be required to get a complete picture of predator activities.  This is a unique opportunity to work in an area where piscivores are still relatively abundant and interactions between predators and prey can be readily observed.