higher average densities (3.0 ind m
-2 ) than Barco Hundido site (0.6 ind m
-2 ), the
Charles Darwin Research Station (0.5 ind m
-2 ) and Punta Núñez (0.3 ind m
-2 ).
Within all sites, density decreased over time. This result was explained by the
movement of sea urchins. It was concluded that T. depressus in intertidal sites
moved from shallow to deeper areas, probably to find more stable environmental
conditions. On the other hand, there was no clear direction or pattern of movement
in subtidal sites. Brandt and Guarderas (2002) described abundance of T. depressus
around the Galápagos Archipelago. The highest densities were found in Floreana
(0.9 ind m
-2 ) and in the southeastern islands (Española and San Cristóbal with
0.6 ind m
-2 ) while the lowest values were found on Fernandina (with
0.05 ind m
-2 ). T. depressus showed significant density patterns associated with
depth, suggesting that it prefers shallow habitats less than 10 m depth (Brandt and
Guarderas 2002).
Size Structure, Growth and Gonad Production
Tripneustes depressus is the largest sea urchin species in the Galápagos Islands
(Hickman 1998). Size structure data for T. depressus is available for more sampling
periods than for any other sea urchin species in the archipelago. Population surveys
done in 1996 showed that the size frequency distribution had two peaks: (1) 7.0 cm,
and (2) 12.0 cm. In 1997, the size-frequency distribution had a single mode
(12.5 cm) and a mean size of 11.5 cm. Consequently the former surveys may have
presented a bimodal shape due to a smaller sample size (Luna 2000). In 1998,
population surveys conducted in the western islands of the archipelago, showed a
mass recruitment event (Brandt and Guarderas 2002). In 1999, intensive sampling
was conducted around three islands of the archipelago (Española, Floreana and San
Cristóbal). The results showed a bimodal size frequency distribution for each island
(Brandt and Guarderas 2002). On the other hand, the information for the 2000–2001
periods indicates that the mean test diameter around the archipelago was
10.0 ± 0.04 cm, with a range of 1–14 cm. The size frequency distribution had two
peaks (5.0 and 9.5 cm). Test diameter was significantly different (H = 499.9,
p \ 0.0001, df = 6) between regions/islands (Brandt and Guarderas 2002)
Guarderas (2000) determined growth parameters for T. depressus using modal
progression of size-frequency distributions of monthly data (September 1998 to
September 1999) from four sites in Santa Cruz Island (El Barranco, Barco Hundido, Muelle de la Estación Charles Darwin and Punta Núñez). Data from all sites
were pooled to estimate population parameters. Analyses and processing of length
data were conducted using the Length Frequency Distribution Analysis (LFDA
Version 4.01), from the FISAT (FAO/ICLARM) package. Growth rate K (1 - L
(t)/L?) declines with time as the diameter of the animal reaches the asymptotic
length. Estimation of growth parameters (K, L?) of the Von Bertalanffy growth
curve was done followed by definition of grid boundaries using the methods of
Powell-Wetheral and ELEFAN (Electronic Length Frequency Analysis). The
asymptotic diameter (L ? ) ranged from 144.7 mm in the Punta Núñez site to
178.8 mm Barco Hundido. In the same way, the body growth rate coefficient (K)
204
J. Sonnenholzner et al.
-2 ) than Barco Hundido site (0.6 ind m
-2 ), the
Charles Darwin Research Station (0.5 ind m
-2 ) and Punta Núñez (0.3 ind m
-2 ).
Within all sites, density decreased over time. This result was explained by the
movement of sea urchins. It was concluded that T. depressus in intertidal sites
moved from shallow to deeper areas, probably to find more stable environmental
conditions. On the other hand, there was no clear direction or pattern of movement
in subtidal sites. Brandt and Guarderas (2002) described abundance of T. depressus
around the Galápagos Archipelago. The highest densities were found in Floreana
(0.9 ind m
-2 ) and in the southeastern islands (Española and San Cristóbal with
0.6 ind m
-2 ) while the lowest values were found on Fernandina (with
0.05 ind m
-2 ). T. depressus showed significant density patterns associated with
depth, suggesting that it prefers shallow habitats less than 10 m depth (Brandt and
Guarderas 2002).
Size Structure, Growth and Gonad Production
Tripneustes depressus is the largest sea urchin species in the Galápagos Islands
(Hickman 1998). Size structure data for T. depressus is available for more sampling
periods than for any other sea urchin species in the archipelago. Population surveys
done in 1996 showed that the size frequency distribution had two peaks: (1) 7.0 cm,
and (2) 12.0 cm. In 1997, the size-frequency distribution had a single mode
(12.5 cm) and a mean size of 11.5 cm. Consequently the former surveys may have
presented a bimodal shape due to a smaller sample size (Luna 2000). In 1998,
population surveys conducted in the western islands of the archipelago, showed a
mass recruitment event (Brandt and Guarderas 2002). In 1999, intensive sampling
was conducted around three islands of the archipelago (Española, Floreana and San
Cristóbal). The results showed a bimodal size frequency distribution for each island
(Brandt and Guarderas 2002). On the other hand, the information for the 2000–2001
periods indicates that the mean test diameter around the archipelago was
10.0 ± 0.04 cm, with a range of 1–14 cm. The size frequency distribution had two
peaks (5.0 and 9.5 cm). Test diameter was significantly different (H = 499.9,
p \ 0.0001, df = 6) between regions/islands (Brandt and Guarderas 2002)
Guarderas (2000) determined growth parameters for T. depressus using modal
progression of size-frequency distributions of monthly data (September 1998 to
September 1999) from four sites in Santa Cruz Island (El Barranco, Barco Hundido, Muelle de la Estación Charles Darwin and Punta Núñez). Data from all sites
were pooled to estimate population parameters. Analyses and processing of length
data were conducted using the Length Frequency Distribution Analysis (LFDA
Version 4.01), from the FISAT (FAO/ICLARM) package. Growth rate K (1 - L
(t)/L?) declines with time as the diameter of the animal reaches the asymptotic
length. Estimation of growth parameters (K, L?) of the Von Bertalanffy growth
curve was done followed by definition of grid boundaries using the methods of
Powell-Wetheral and ELEFAN (Electronic Length Frequency Analysis). The
asymptotic diameter (L ? ) ranged from 144.7 mm in the Punta Núñez site to
178.8 mm Barco Hundido. In the same way, the body growth rate coefficient (K)
204
J. Sonnenholzner et al.