Sea urchin predation mortality rates

Brief description

Mortality rates of the sea urchin Heliocidaris erythrogramma where estimated under experimental conditions to determine the importance of natural and predation mortality in structuring sea urchin populations. Data was obtained from tethering, tagging and caging experiments at four sites within Mercury Passage and the Derwent Estuary in eastern Tasmania.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: 1. Tethering experiment
Mortality of tethered sea urchins was examined in both regions on the east coast of Tasmania between October 2000 and January 2001. Within each region two areas of rocky reef were selected, one supporting a high density of predators and low density of sea urchins (no-take reserve) and an adjacent area open to fishing with relatively few predators and relatively more sea urchins. Within each area two 20 m transects, separated by approximately 50 m, were deployed randomly at a depth of 6–8 m. Along each transect were placed 40 tethered sea urchins comprising 20 juveniles (test diameter 20–60 mm) and 20 adults (test diameter 65–100 mm). Within each region, all animals were collected nearby the experimental area.
Mortality was assessed over a 14-day period by relocating tethers and recording the state of the attached sea urchin. Tethers with no sea urchin attached were recorded as mortality events. All tethers were accounted for at the end of the experiment.

2. Size-specific predation of sea urchins by rock lobsters
Size-specific predation by rock lobsters was examined in a caging experiment conducted in Mercury Passage outside the marine reserve in January 2001. Cages were constructed from a steel frame (1 m × 1 m at the base and 0.5 m high) covered in 5 mm plastic mesh. Secured around the inside bottom edge of each cage was a flexible 150 mm mesh skirt bordered by 8 mm chain to prevent rock lobsters escaping from the cages during the experimental period.

Each cage was placed on a suitable patch of reef so that the steel frame of the cage was resting on relatively flat substratum. Five sea urchins from each of the three size classes (20–60 mm, 61–80 and 81–100 mm test diameter) were placed in each cage, and cages were left overnight to ensure no animals could escape from the cages. A single rock lobster (J. edwardsii) of a designated size class was then added to each ‘treatment’ cage while ‘control’ cages did not contain rock lobsters. There were 3 replicate control cages, and 3 replicate treatment cages of each size class of rock lobster. Size classes of rock lobsters were denoted as juvenile (J; carapace length (CL) < 100 mm), adult class 1 (A1; CL 110 mm), and adult class 2 (A2; CL > 120 mm). On the east coat of Tasmania male J. edwardsii do not feed during their annual moult in November and December, with feeding rates of the rock lobsters thought to increase in the post moult period. The experiment was established early in January 2001 using post moult male rock lobsters to ensure that rock lobsters would be feeding.

3. Tagging experiment — sea urchins marked but not tethered
The method used to tag sea urchins was identical to that for tethering the sea urchins (above) except that no weight was attached to the free end of the wire tag. Sixty tagged sea urchins were placed randomly on each of the four transects in each region, comprising 30 juvenile (test diameter 20–60 mm) and 30 adult (test diameter 65–100 mm) sea urchins. Tagged animals were placed in positions to ensure that individuals could seek shelter in crevices. Sea urchin mortality was assessed weekly for one month by relocating tags and recording the condition of the attached sea urchin. Recovered tags that were not attached to an intact sea urchin or fragment of sea urchin test were considered to be mortalities since our pilot studies found that > 90% of tags were retained by sea urchins held for one month in an aquarium. Unrecovered tags were not considered to be mortalities and were excluded from analyses.

4. Relative effects of fish and rock lobsters as predators of sea urchins
Treatments were designed to examine all possible combinations of the presence (+) and absence (−) of rock lobsters (L) and demersal fish (F), (Fig. 2). Three replicate cages of each of the four treatments and three replicate unmanipulated control plots were positioned randomly at ca. 8 m depth on the rocky reef at Magistrates Point (42° 34′ S, 148° 03′ E).
Each experimental plot was then stocked with a total of fifteen sea urchins from the surrounding reef to give an overall sea urchin density of 1.7 individuals m− 2. Five sea urchins were selected from each of the three size classes, viz. juvenile (J) 40–60 mm, small adult (A1) 61–80 mm, and adult (A2) 81–100 mm. Sea urchins were allowed to acclimate to the experimental plots for 24 h before the addition of rock lobsters. The number of sea urchins from each size class was recorded after the acclimation period, and all individuals were accounted for. Sea urchin mortality in the experimental plots was assessed weekly over a two-month period.

5. Size–frequency distributions of sea urchin populations within Mercury Passage
The size–structure of five sea urchin populations from areas open and closed to exploitation of rock lobster and demersal fish in Mercury Passage was compared. Sea urchins were collected randomly from rocky reefs at three locations open to harvesting (Quarry Point 42° 34′ S, 147° 54′ E; Stapleton Point 42° 35′ S, 147° 55′ E; Lords Bluff 42° 31′ S, 147° 59′ E), and from two locations within the Maria Island Marine Reserve closed to fishing (Magistrates Point 42° 34′ S, 148° 03′ E; Howells Point 42° 36′ S, 148° 02′ E). At each of the five locations all sea urchins were collected from 10 randomly placed 16-m2 quadrats (mid 2000) and the test diameter (TD) of each individual measured to the nearest millimetre using knife-edge callipers.

Notes

Credit
Australian Research Council (ARC)

Credit
School of Zoology, University of Tasmania

Purpose
Quantify sea urchin mortality rates in areas where predator abundance are high (inside marine reserves) and in areas open to exploitation.