Year-to-year changes of vertical temperature distribution in the California Current region: 1954 to 1986

Studies by Enfield and Allen (1980), McLain et al (1985), and others have shown that anomalously warm years in the northern coastal California Current correspond to El Niño conditions in the equatorial Pacific Ocean. Ocean model studies suggest a mechanical link between the northern coastal California Current and the equatorial ocean through long waves that propagate cyclonically along the ocean boundary (McCreary 1976; Clarke 1983; Shriver et al 1991). However, distinct observational evidence of such an oceanic connection is not extensive. Much of the supposed El Niño variation in temperature and sea level data from the coastal California Current region can be associated with the effects of anomalously intense north Pacific atmospheric cyclogenesis, which is frequently augmented during El Niño years (Wallace and Gutzler 1981; Simpson 1983; Emery and Hamilton 1984). This study uses time series of ocean temperature data to distinguish between locally forced effects, initiated by north Pacific atmospheric changes, and remotely forced effects, initiated by equatorial Pacific atmospheric changes related to El Niño events.

Diel vertical migration of the bigeye thresher shark (Alopias superciliosus), a species possessing orbital retia mirabilia

The bigeye thresher shark (Alopias superciliosus, Lowe 1841) is one of three sharks in the family Alopiidae, which occupy pelagic, neritic, and shallow coastal waters throughout the altropics and subtropics (Gruber and Compagno, 1981; Castro, 1983). All thresher sharks possess an elongated upper caudal lobe, and the bigeye thresher shark is distinguished from the other alopiid sharks by its large upward-looking eyes and grooves on the top of the head (Bigelow and Schroeder, 1948). Our present understanding of the bigeye thresher shark is primarily based upon data derived from specimens captured in fisheries, including knowledge of its morphological features (Fitch and Craig, 1964; Stillwell and Casey, 1976; Thorpe, 1997), geographic range as far as it overlaps with fisheries (Springer, 1943; Fitch and Craig, 1964; Stillwell and Casey, 1976; Gruber and Compagno, 1981; Thorpe, 1997), age, growth and maturity (Chen et al., 1997; Liu et al., 1998), and aspects of its reproductive biology (Gilmore, 1983; Moreno and Moron, 1992; Chen et al., 1997).

Distribution, demography, and discard mortality of crabs caught as bycatch in an experimental pot fishery for toothfish (Dissostichus eleginoides) in the South Atlantic

Between March 2000 and April 2001 two commercial fishing vessels fished for toothfish (Dissostichus eleginoides) off South Georgia using pots. A significant number of lithodid crabs (three species of Paralomis spp.) were caught as bycatch. Paralomis spinosissima occurred in shallow water, generally shallower than 700 m. Paralomis anamerae, not previously reported from this area and therefore representing a considerable southerly extension in the reported geographic range of this species, had an intermediate depth distribution from 400 to 800 m. Paralomis formosa was present in shallow waters but reached much higher catch levels (and, presumably, densities) between 800 and 1400 m. Differences were also noted in depth distribution of the sexes and size of crabs. Depth, soak time, and area were found to significantly influence crab catch rates. Few crabs (3% of P. spinosissima and 7% of P. formosa) were males above the legal size limit and could therefore be retained. All other crabs were discarded. Most crabs (>99% of P. formosa, >97% of P. spinosissima, and >90% of P. anamerae) were lively on arrival on deck and at subsequent discard. Mortality rates estimated from re-immersion experiments indicated that on the vessel where pots were emptied directly onto the factory conveyor belt 78–89% of crabs would survive discarding, whereas on the vessel where crabs were emptied down a vertical chute prior to being sorted, survivorship was 38–58%. Of the three, P. anamerae was the most vulnerable to handling onboard and sub-sequent discarding. Paralomis spinosissima seemed more vulnerable than P. formosa.

Horizontal and vertical movements of juvenile bluefin tuna (Thunnus thynnus) in relation to oceanographic conditions of the western North Atlantic, determined with ultrasonic telemetry

We employed ultrasonic transmitters to follow (for up to 48 h) the horizontal and vertical movements of five juvenile (6.8–18.7 kg estimated body mass) bluefin tuna (Thunnus thynnus) in the western North Atlantic (off the eastern shore of Virginia). Our objective was to document the fishes’ behavior and distribution in relation to oceanographic conditions and thus begin to address issues that currently limit population assessments based on aerial surveys. Estimation of the trends in adult and juvenile Atlantic bluefin tuna abundance by aerial surveys, and other fishery-independent measures, is considered a priority. Juvenile bluefin tuna spent the majority of their time over the continental shelf in relatively shallow water (generally less then 40 m deep). Fish used the entire water column in spite of relatively steep vertical thermal gradients (≈24°C at the surface and ≈12°C at 40 m depth), but spent the majority of their time (≈90%) above 15 m and in water warmer then 20°C. Mean swimming speeds ranged from 2.8 to 3.3 knots, and total distance covered from 152 to 289 km (82–156 nmi). Because fish generally remained within relatively con-fined areas, net displacement was only 7.7–52.7 km (4.1–28.4 nmi). Horizontal movements were not correlated with sea surface temperature. We propose that it is unlikely that juvenile bluefin tuna in this area can detect minor horizontal temperature gradients (generally less then 0.5°C/km) because of the steep vertical temperature gradients (up to ≈0.6°C/m) they experience during their regular vertical movements. In contrast, water clarity did appear to influence behavior because the fish remained in the intermediate water mass between the turbid and phytoplankton-rich plume exiting Chesapeake Bay (and similar coastal waters) and the clear oligotrophic water east of the continental shelf.

Vertical and horizontal movements of southern bluefin tuna (Thunnus maccoyii) in the Great Australian Bight observed with ultrasonic telemetry

The vertical and horizontal movements of southern bluefin tuna (SBT), Thunnus maccoyii, in the Great Australian Bight were investigated by ultrasonic telemetry. Between 1992 and 1994, sixteen tuna were tracked for up to 49 h with depth or combined temperature-depth transmitting tags. The average swimming speeds (measured over the ground) over entire tracks ranged from 0.5 to 1.4 m/s or 0.5 to 1.4 body lengths/s. The highest sustained swimming speed recorded was 2.5 m/s for 18 hours. Horizontal movements were often associated with topographical features such as lumps, reefs, islands and the shelf break. They spent long periods of time at the surface during the day (nearly 30%), which would facilitate abundance estimation by aerial survey. At night, they tended to remain just below the surface, but many remained in the upper 10 m throughout the night. SBT were often observed at the thermocline interface or at the surface while travelling. A characteristic feature of many tracks was sudden dives before dawn and after sunset during twilight, followed by a gradual return to their original depth. It is suggested that this is a behavior evolved to locate the scattering layer and its associated prey when SBT are in waters of sufficient depth. SBT maintained a difference between stomach and ambient temperature of up to 9°C.

On the relative utility of different methods to increase the vertical height in an otter trawl

Comparative fishing experiments to study the relative utility of different methods for increased vertical spread of bottom trawl and the availability of off bottom fishes in the region were made using gussets, kite, separate float line and side panels on a two seam net. The catch rates as well as composition of fish were studied. The opening of the trawl mouth, both horizontally and vertically, under different operating gears and towing tension on warps were measured and estimated for comparison purposes. Better catch rate with good quality fishes was obtained with the gear operated with separate float line. With kite, the vertical spread was increased with less catch indicating poor concentration of off bottom fishes.

Vertical distribution of molluscs in the intertidal area in and around Mumbai, India

Vertical distribution of intertidal molluscs in and around Mumbai had been studied. Each species has an upper and lower limit of distribution along the vertical intertidal gradient and are concentrated at particular levels or zones where they find optimum living conditions. Zonation of the intertidal area with reference to molluscs at rocky shores of TIFR, Bandstand and NCPA has similarities. However, there is no similarity in zonation among rocky, sandy and muddy shores. Rocky intertidal zones are more diverse and dense in terms of molluscs. The mid and lower littoral zones have rich diversity. The upper littoral zone at some sites, especially Girgaon chowpatty is totally devoid of molluscs due to anthropogenic activities. Gafrarium divaricatum, Nerita oryzarum, N. polita and Neritina crepidularia have established themselves in all three marked zones, indicating their power to adjust with the wide fluctuations in surrounding environmental conditions.

Vertical temperature structure of 250 m. water layer around Sri Lanka at the tail-end of the north-east monsoon

Data collected during cruises of the Hoyo Maru in Jan-Mar 1975 are analysed. Data is tabulated to show vertical temp profile, surface water temp and temp gradient. Each of these features is discussed. Thermoclines are shown to be established off the coast of India, their depth varying according to time of year. Upwelling off the Cochin coast is discussed. This occurs during Oct-Nov. Surface temp is considerably influenced by the north-east monsoon. The 'clockwise current' (or 'transparent current) characterised by high salinity, high transparency, and rich nutrient conchs, and which prevails in Jan-May brings oceanic water into the Bay of Bengal and sweeps along the Ceylon coast.

On the vertical distribution of seers and other commercially important fishes in the surface drift nets

This note highlights the author's attempts to determine the vertical height for catching seers (Scomberomorus sp.) and other commercially important fish off Kakinada coast.

Vertical distribution of marine wood boring and fouling organisms from the estuarine areas of the south west coast of India

Vertical distribution of marine wood boring and fouling organisms from three different estuarine areas namely, the Ernakulam channel in the Cochin backwaters, Ayiramthengu in the Kayamkulam Lake and Neendakara in the Asthamudi Lake during the post-monsoon, the pre-monsoon and the monsoon periods is presented. The boring organisms noticed during the present study were Martesia striata, Teredo furcifera, Nausitora hedleyi and Sphaeroma terebrans. The dominant fouling organisms were Balanus amphitrite amphitrite, calcareous worms and Modiolus sp. Algae and diatoms were very common on the sub-tidal panels during the monsoon. The incidence of Teredo, Nausitora and calcareous tube worms were significantly high on the bottom panels. Sphaeroma, Balanus and Modiolus occurred in greater numbers on the intertidal panels.