Feasibility of adopting integrated rice-cum-fish culture system to enhance the development of conventional aquaculture participation in Niger State

In a survey conducted to find out the status of integrated rice-cum-fish culture in Niger State, Nigeria, 0.37 ha of Fadama wetlands was utilized for rice-cum-fish culture and at experimental stage. In the case study of this rice-cum-fish model, the Nile Tilapia (Oreochromis niloticus) was involved. The result was that 1,4720 kg/ha/yr could be produced using chick manure application under rice-cum-fish culture model. The available records reveal that 233,079 ha out of 495,000 ha of estimated Fadama in Niger State was used for rice cultivation in 1997. If 233,079 ha were to be used for integrated rice-cum fish culture, it is estimated that 343,092 mt of fish (Oreochromis niloticus) could be produced per year. The fish demand in Niger State in 2002 was 50,000 mt. The NPK application under rice-cum-fish production gave the best rice production estimated at 43,968.0 kg/ha/yr. The percentage increase in rice yield as well as increase in net income due to introduction of fish was 10.1 % and 54.4% respectively. The culture system is therefore recommended for adoption towards greater participation in aquaculture development by the farmers

Assessment of habitat quality for juvenile California halibut (Paralichthys californicus) in a seasonally arid estuary

We evaluated habitat quality for juvenile California halibut (Paralichthys californicus) in a Pacific Coast estuary lacking in strong salinity gradients by examining density, recent otolith growth rates, and gut fullness levels of wild-caught and caged juveniles for one year. Juveniles <200 mm standard length were caught consistently in the inner, central, and outer sections of the estuary. The density of juveniles was two times higher in the inner estuary during most of the year, consistent with active habitat selection by part of the population. A generalized linear model indicated temperature, sampling time, and the interaction between salinity and temperature were significantly related to density. However, the model explained only 21% of the variance. Gut fullness levels of wild-caught juveniles were highest during the summer, but recent otolith growth rates were not related to temperature. The proportion of individuals feeding successfully indicated that seasonal differences in food availability are more important than spatial variation in prey abundance in driving feeding success. Feeding success of caged fishes was limited, precluding the use of growth rates as indicators of local habitat quality. However, marginal increment widths were reliable indicators of somatic growth at low growth rates over two-week periods. The relatively high growth rates and abundance of small wild-caught juveniles found throughout the estuary indicates that the entire estuary system has the potential for serving as nursery habitat.

Evaluation of rockfish abundance in untrawlable habitat: combining acoustic and complementary sampling tools

Rockfishes (Sebastes spp.) are an important component of North Pacific marine ecosystems and commercial fisheries. Because the rocky, high-relief substrate that rockfishes often inhabit is inaccessible to standard survey trawls, population abundance assessments for many rockfish species are difficult. As part of a large study to classify substrate and compare complementary sampling tools, we investigated the feasibility of using an acoustic survey in conjunction with a lowered stereo-video camera, a remotely operated vehicle, and a modified bottom trawl to estimate rockfish biomass in untrawlable habitat. The Snakehead Bank south of Kodiak Island, Alaska, was surveyed repeatedly over 4 days and nights. Dusky rockfish (S. variabilis), northern rockfish (S. polyspinis), and harlequin rockfish (S. variegatus) were the most abundant species observed on the bank. Backscatter attributed to rockfish were collected primarily near the seafloor at a mean height off the bottom of 1.5 m. Total rockfish backscatter and the height of backscatter off the bottom did not differ among survey passes or between night and day. Biomass estimates for the 41 square nautical-mile area surveyed on this small, predominantly untrawlable bank were 2350 metric tons (t) of dusky rockfish, 331 t of northern rockfish, and 137 t of harlequin rockfish. These biomass estimates are 5–60 times the density estimated for these rockfish species by a regularly conducted bottom trawl survey covering the bank and the surrounding shelf. This finding shows that bottom trawl surveys can underestimate the abundance of rockfishes in untrawlable areas and, therefore, may underestimate overall population abundance for these species.

Depredation of catch by bottlenose dolphins (Tursiops truncatus) in the Florida king mackerel (Scomberomorus cavalla) troll fishery

We documented depredation by bottlenose dolphins (Tursiops truncatus) in the Florida king mackerel (Scomberomorus cavalla) troll fishery. Between March and June 2003, we conducted 26 interviews of charter and commercial fishermen in Islamorada, Florida, and 23 along Florida’s east coast from Fort Pierce south to Lake Worth Inlet. All fishermen indicated they had observed bottlenose dolphins depredating bait or catch—king mackerel being the species most often taken by dolphins. During on-board observations of depredation between March and June 2003, we found that dolphins took 6% of king mackerel caught by charter fishermen and 20% of fish caught by commercial fishermen. We concluded that depredation by bottlenose dolphin occurs commonly in this fishery and has the potential to incur a significant economic cost to king mackerel fishermen. To address this concern, we conducted preliminary tests of a gear modification designed to reduce depredation in the king mackerel fishery between December 2003 and January 2004. These tests demonstrated that a modification to the outrigger planer will successfully deter bottlenose dolphins from engaging in depredation, without causing a reduction in ca

Abundance and distribution of California sea lions (Zalophus californianus) in central and northern California during 1998 and summer 1999

The abundance and distribution of California sea lions (Zalophus californianus) in central and northern California was studied to allow future evaluation of their impact on salmonids, the ecosystem, and f isheries. Abundance at-sea was estimated by using the strip transect method from a fixed-wing aircraft with a belly viewing port. Abundance on land was estimated from 126-mm-format aerial photographs of animals at haulouts between Point Conception and the California−Oregon border. The sum of these two estimates represented total abundance for central and northern California. Both types of survey were conducted in May−June 1998, September 1998, December 1998, and July 1999. A haulout survey was conducted in July 1998. The greatest number of sea lions occurred near Monterey Bay and San Francisco Bay for all surveys. Abundance was high in central and northern California in 1998 when warm water from the 1997−98 El Niño affected the region and was low in July 1999 when cold water La Niña conditions were prevalent. At-sea abundance estimates in central and northern California ranged from 12,232 to 40,161 animals, and haulout abundance was 13,559 to 36,576 animals. Total abundance of California sea lions in central and northern California was estimated as 64,916 in May−June 1998, 75,673 in September 1998, 56,775 in December 1998, and 25,791 in July 1999. The proportion of total abundance to animals hauled-out for the four complete surveys ranged from 1.77 to 2.13, and the mean of 1.89 was used to estimate a total abundance of 49,697 for July 1998. This multiplier may be applicable in the future to estimate total abundance of California sea lions off central and northern California if only the abundance of animals at haulout sites is known.

Growth, recruitment, and abundance of juvenile striped mullet (Mugil cephalus) in South Carolina estuaries

Growth, recruitment, and abundance of young-of-the-year (YOY) striped mullet (Mugil cephalus L.) in estuarine habitats in South Carolina from 1998 to 2000 were examined and compared to historical data (1986–91) of growth, recruitment, and abundance. Daily growth increments from the sagittal otoliths of juvenile striped mullet were validated by using fish immersed in oxytetracycline hydrochloride (OTC) for five hours from the Charleston Harbor Estuary system. The distribution of back-calculated birthdates indicated that striped mullet spawn from October to late April and estuarine recruitment occurs from January through May. Juveniles were more abundant in mesohaline and polyhaline salinity regimes but were found throughout the estuary. Juvenile growth after recruitment into the estuary can be described by the relationship Total length (mm) = 0.341 (Age)1.04 (r2=0.741, P=0.001). Growth of juveniles according to the analysis of size-frequency data from historical surveys (1986 to 1991) in the same estuaries gave the relationship Total length (mm) = 8.77 (month)1.12 (r2=0.950, P=0.001). The similarity in the growth curves for both groups of fish suggests that juvenile striped mullet in South Carolina have consistent annual growth during the first year of life.