Estimating species and size composition of rockfishes to verify targets in acoustic surveys of untrawlable areas

Rockfish (Sebastes spp.) biomass is difficult to assess with standard bottom trawl or acoustic surveys because of their propensity to aggregate near the seafloor in highrelief areas that are inaccessible to sampling by trawling. We compared the ability of a remotely operated vehicle (ROV), a modified bottom trawl, and a stereo drop camera system (SDC) to identify rockfish species and estimate their size composition. The ability to discriminate species was highest for the bottom trawl and lowest for the SDC. Mean lengths and size distributions varied among the gear types, although a larger number of length measurements could be collected with the bottom trawl and SDC than with the ROV. Dusky (S. variabilis), harlequin (S. variegatus), and northern rockfish (S. polyspinis), and Pacific ocean perch (S. alutus) were the species observed in greatest abundance. Only dusky and northern rockfish regularly occurred in trawlable areas, whereas these two species and many more occurred in untrawlable areas. The SDC was able to resolve the height of fish off the seafloor, and some of the rockfish species were observed only near the seafloor in the acoustic dead zone. This finding is important, in that fish found exclusively in the acoustic dead zone cannot be assessed acoustically. For these species, methods such as bottom trawls, long-lines, or optical surveys using line transect or area swept methods will be the only adequate means to estimate the abundance of these fishes. Our results suggest that the selection of appropriate methods for verifying targets will depend on the habitat types and species complexes to be examined.

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.

Growth, mortality and recruitment of Nile perch Lates niloticus (L. Centropomidae) in the Nyanza Gulf of Lake Victoria: an evaluation update

A reassessment of the estimates of growth, mortality and recruitment patterns of Nile Perch, Lates niloticus was made based on data from commercial landings collected during the Catch Assessment Survey Programme. Two sets of length frequency data, one each from beach seining and hook and line fisheries, were analyzed. Values of L8 = 169 and 230 (cm TL) and K= 0.18 yr-1 and 0.195 yr-1 were obtained. The total mortality estimates from the catch curve analysis were Z = 0.72 yr-1 and 0.94 yr-1, respectively, with a natural mortality M of about 0.35 for a mean environmental temperature of 27oC. The highest peak for recruitment was in November, December and January with a minor one in June, indicating recruitment of two cohorts per year. These results are discussed and compared to previously available information on L. niloticus in Lake Victoria.

Larval development of estuary perch (Macquaria colonorum) and Australian bass (M. novemaculeata) (Perciformes: Percichthyidae), and comments on their life history

Morphological development of the larvae and small juveniles of estuary perch (Macquaria colonorum) (17 specimens, 4.8−13.5 mm body length) and Australian bass (M. novemaculeata) (38 specimens, 3.3−14.1 mm) (Family Percichthyidae) is described from channel-net and beach-seine collections of both species, and from reared larvae of M. novemaculeata. The larvae of both are characterized by having 24−25 myomeres, a large triangular gut (54−67% of BL) in postflexion larvae, small spines on the preopercle and interopercle, a smooth supraocular ridge, a small to moderate gap between the anus and the origin of the anal fin, and distinctive pigment patterns. The two species can be distinguished most easily by the different distribution of their melanophores. The adults spawn in estuaries and larvae are presumed to remain in estuaries before migrating to adult freshwater habitat. However, larvae of both species were collected as they entered a central New South Wales estuary from the ocean on flood tides; such transport may have consequences for the dispersal of larvae among estuaries. Larval morphology and published genetic evidence supports a reconsideration of the generic arrangement of the four species currently placed in the genus Macquaria.

Pacific Hagfish, Eptatretus stouti, and Black Hagfish, E. deani: The Oregon Fishery and Port Sampling Observations, 1988-92

In 1988, the Oregon Department of Fish and Wildlife began sampling and monitoring the development of a new fishery for Pacific hagfish, Eptatretus stouti, and black hagfish, E. deani. Hagfish landings by Oregon trap vessels have ranged from 11,695 kg in 1988 to 340,774 kg in 1992. Whole frozen fish were shipped to South Korea for the "eel skin" leather market. From 1988 through 1989, I sampled 924 Pacific hagfish and 897 black hagfish from commercial and research catches. Mean length of fish sampled from commercial landings was 39.6 cmf or Pacific hagfish and 34.5 cm for black hagfish. Weight-length relationships (W=aLb) were calculated for males and females of both species. Fifty percent maturity for male and female Pacific hagfish was 35 cm and 42 cm, respectively, while 50% maturityf or male and female black hagfish was 34 cm and 38 cm, respectively. Examination of gonads for both species indicated that spawning either occurs throughout the year or the spawning period is protracted. Mature females of both species had from one to three distinct sizes of eggs, but they usually carried only one group of eggs over 5 mm in length. Mature Pacific hagfish females averaged 28 eggs over 5 mm in length, and black hagfish females averaged 14 eggs over 5 mm in length. Hermaphroditism was found in 0.2% of the Pacific hagfish examined.

A Comparison of Two Stratification Schemes Used in Sampling Canadian Atlantic Cod, Gadus morhua

Sampling is a key element in the assessment of any fish stock. It is often one of the most expensive activities of the management process; thus, improved efficiency can result in significant cost savings. In most cases a two-phase sampling strategy is employed. Two commonly used versions of such stratified random schemes were simulated using a test population based on Atlantic cod, Gadus morhua. A 1 otolith per 1 cm length frequency currently used for many flatfish and some smaller gadoids and a 3 otolith per 3 cm length frequency currently used for many of the larger gadoids. No difference was detected in the age composition or mean length at age for either scheme; however, 10 percent fewer otoliths were collected in 1 for 1 sampling than 3 for 3. There was an improvement of between 30 and 60 percent in the coefficient of variation of the estimated catch numbers at age using the 1 for 1 compared with the 3 for 3 stratified sampling. For these reasons and other operational considerations, the 1 for 1 stratified random design of sampling appears to be superior.