537
12 Species Case Studies
Rabbitfi sh spawn year round with the main spawning season in Palau between
February and June, and occasionally a secondary peak between September and
November; detailed studies conducted over various time periods provide fi ner temporal scale information on fi sh densities and biomass. The smallest sexually mature
males were measured at 12.5 cm SL (standard length) for males and 15 cm SL for
females (AHK unpublished data 2004). Gonads from a confi scated catch (taken during closed spawning season) represented 11% of the total body weight for 18 females
and 8% for 33 males. The sizes of fi sh caught from PSAs in 1990–1991 (mean fork
length-FL = 18.6 cm, sd = 2.1 cm, n = 526) were similar to those caught in 2005 (mean
FL =18.6, sd = 1.9 cm, n = 82), but in 2005 there were many fewer adult fi sh greater
than FL 23.0 cm, implying loss of the largest size category of spawning fi sh.
Spawning of dusky rabbitfi sh has not been witnessed in the wild, but spawning
activity was observed in the tanks in the early morning. Males actively court females
who eventually go to the bottom of sandy areas and release a batch of eggs which
the male fertilizes (Thomas Taro, personal communication 2008). Johannes ( 1981 )
described from fi sher interviews how, just prior to spawning, fi sh tend to mill in
slow tight circles and form pairs. Fishers stated that rabbitfi sh migrate from adjacent
areas to spawn in Airai Bay in southeastern Babeldaob, the main island of Palau and
it is thought that there are resident populations as well as migratory populations
within the bay (Clarence Kitalong, personal communication 2008). Pre-spawners
are also known to release gametes in the nets or into the boats after capture.
In aquaculture studies in Palau females released about 500,000 adhesive eggs at
a time (Drew 1973 ) that are denser than seawater and settle to the substrate where
they are scattered on the bottom (De los Santos et al. 2008 ). Broodstock spawned
year round (De los Santos et al. 2008 ) with egg production on the 4th–8th day after
full moon (De los Santos et al. 2008 ). New vitellogenic oocytes appear on day 2
after the fi rst spawning and are fully mature by day 30. When a greater percentage
of the most advanced oocytes fully develop, they form a batch and separate from the
adjacent group of smaller pre-vitellogenic oocytes, indicating that the dusky rabbitfi sh is a multiple spawner with an ovary belonging to the group-synchronous type
of oocyte development. Batch fecundity ranged from 0.52 to 2.56 million eggs.
Fecundity (F) increased exponentially (F = .0536854 L
5.07292 ) with fork length (Hoque
et al. 1999 ) . One female in captivity spawned repeatedly in successive months
(Hasse et al. 1977 ) . Eggs develop within 32 h, phytoplankton is found in larval guts
within a day of hatching. The planktonic stage is 21–23 days and newly metamorphosed fi sh average 23 mm in standard length.
12.22.3 Commercial/Subsistence Importance
Dusky rabbitfi sh have long supported subsistence and, more recently, modest commercial fi sheries. While data are limited, annual commercial production for dusky
rabbitfi sh from 1976 to 2006 indicate 5-year cycles, with peak catches during 1980,
1985 and 1990 and the highest reported landings of over 30 metric tonnes in 1990
12 Species Case Studies
Rabbitfi sh spawn year round with the main spawning season in Palau between
February and June, and occasionally a secondary peak between September and
November; detailed studies conducted over various time periods provide fi ner temporal scale information on fi sh densities and biomass. The smallest sexually mature
males were measured at 12.5 cm SL (standard length) for males and 15 cm SL for
females (AHK unpublished data 2004). Gonads from a confi scated catch (taken during closed spawning season) represented 11% of the total body weight for 18 females
and 8% for 33 males. The sizes of fi sh caught from PSAs in 1990–1991 (mean fork
length-FL = 18.6 cm, sd = 2.1 cm, n = 526) were similar to those caught in 2005 (mean
FL =18.6, sd = 1.9 cm, n = 82), but in 2005 there were many fewer adult fi sh greater
than FL 23.0 cm, implying loss of the largest size category of spawning fi sh.
Spawning of dusky rabbitfi sh has not been witnessed in the wild, but spawning
activity was observed in the tanks in the early morning. Males actively court females
who eventually go to the bottom of sandy areas and release a batch of eggs which
the male fertilizes (Thomas Taro, personal communication 2008). Johannes ( 1981 )
described from fi sher interviews how, just prior to spawning, fi sh tend to mill in
slow tight circles and form pairs. Fishers stated that rabbitfi sh migrate from adjacent
areas to spawn in Airai Bay in southeastern Babeldaob, the main island of Palau and
it is thought that there are resident populations as well as migratory populations
within the bay (Clarence Kitalong, personal communication 2008). Pre-spawners
are also known to release gametes in the nets or into the boats after capture.
In aquaculture studies in Palau females released about 500,000 adhesive eggs at
a time (Drew 1973 ) that are denser than seawater and settle to the substrate where
they are scattered on the bottom (De los Santos et al. 2008 ). Broodstock spawned
year round (De los Santos et al. 2008 ) with egg production on the 4th–8th day after
full moon (De los Santos et al. 2008 ). New vitellogenic oocytes appear on day 2
after the fi rst spawning and are fully mature by day 30. When a greater percentage
of the most advanced oocytes fully develop, they form a batch and separate from the
adjacent group of smaller pre-vitellogenic oocytes, indicating that the dusky rabbitfi sh is a multiple spawner with an ovary belonging to the group-synchronous type
of oocyte development. Batch fecundity ranged from 0.52 to 2.56 million eggs.
Fecundity (F) increased exponentially (F = .0536854 L
5.07292 ) with fork length (Hoque
et al. 1999 ) . One female in captivity spawned repeatedly in successive months
(Hasse et al. 1977 ) . Eggs develop within 32 h, phytoplankton is found in larval guts
within a day of hatching. The planktonic stage is 21–23 days and newly metamorphosed fi sh average 23 mm in standard length.
12.22.3 Commercial/Subsistence Importance
Dusky rabbitfi sh have long supported subsistence and, more recently, modest commercial fi sheries. While data are limited, annual commercial production for dusky
rabbitfi sh from 1976 to 2006 indicate 5-year cycles, with peak catches during 1980,
1985 and 1990 and the highest reported landings of over 30 metric tonnes in 1990