Implications For Stock Management Research Articles

Causes of walleye pollock (Theragra chalcogramma) recruitment decline in the northern Sea of Japan: implications for stock management

Recruitment of the northern Japan Sea stock (JSS) of walleye pollock has been decreasing since around 1990. In this study, I analyzed the factors causing this decrease in recruitment by investigating the relationship between recruitment, spawning stock biomass (SSB) and environmental factors using a generalized additive model (GAM). GAM fit to the data showed the importance of SSB, sea surface temperature (SST), ocean current strength (Tsushima Warm Current) and wind intensity (Asian monsoon) in determining the recruitment. Of these, the relationship between SSB and recruitment was positive and not negatively density-dependent. On the other hand, the recruitment was negatively related to SST and ocean current strength, and a dome-shaped relationship was observed between wind intensity and recruitment. Since around 1990, the values of SST and ocean current strength have mostly been high and that of wind intensity mostly low. In addition, SSB has been decreasing since the late 1990s. It is likely that the recruitment decline of JSS after approximately 1990 has been caused by warm water temperature, strong Tsushima Warm Current and weak Asian monsoon, and that the recent decrease in SSB has amplified this recruitment decline. According to the model’s estimation, a recruitment recovery due to environmental improvement will be highly restricted as long as SSB remains at its current low level. Significant recovery of SSB is urgently needed for JSS.

Campylobacter jejuni inactivation in New Zealand soils

The study was undertaken to determine the inactivation rate of Campylobacter jejuni in New Zealand soils. Farm dairy effluent (FDE) inoculated at c. 10(5) ml(-1) with C. jejuni was applied to intact soil cores at a rate of 2 l m(-2). Four soils were used: Hamilton (granular); Taupo (pumice); Horotiu and Waihou (allophanic). After FDE application cores were incubated at 10 degrees C for up to 32 days. For all four soils all the FDE remained within the cores and at least 99% of C. jejuni were retained in the top 5 cm. Campylobacter jejuni had declined to the limit of detection (two C. jejuni 100 g(-1)) by 25 days in Hamilton and Taupo soils and by 32 days in Waihou soil. In contrast, in Horotiu soil the decline was only three orders of magnitude after 32 days. Simulated heavy rainfall was applied 4 and 11 days after FDE application and only about 1% of the applied C. jejuni were recovered in leachates. This study demonstrated that at least 99% of applied C. jejuni were retained in the top 5 cm of four soils where they survived for at least 25 days at 10 degrees C. Soil retention of C. jejuni is efficient at FDE application rates that prevent drainage losses. The low infectious dose of C. jejuni and its ability to survive up to 25 days have implications for stock management on dairy farms.

Aspects of the stock dynamics and exploitation of cuttlefish, Sepia officinalis (Linnaeus, 1758), in the English Channel

Basic biological parameters of cuttlefish, Sepia officinalis, in the English Channel are described from samples of commercial and research vessel landings made between April 1994 and September 1995. There was a significant difference between the length–weight relationship of male and female cuttlefish. Growth of both sexes was rapid and seasonal during the last 12 months of life. Males grew faster than females, and reached larger overall lengths and weights. Most males reached maturity before the start of their second winter, although the testis continued to develop until spawning took place the following spring. Female maturation began later and was more prolonged such that it was completed towards the end of the second winter. Adults of both sexes spawned after the second winter between late March and July. Commercial landings data showed spawning cuttlefish initially arrived on inshore grounds in the western Channel, but slightly later and in greater numbers on the inshore grounds of the middle and eastern Channel. The weight and value of cuttlefish landings made by UK vessels in the Channel increased greatly between the mid 1980s and the mid 1990s such that cuttlefish are now a major part of the earnings for several fisheries. The most important fisheries were the offshore beam trawls, and inshore otter trawls and nets. The exploitation pattern of each fishery is described from quarterly samples of commercial landings taken between April 1994 and March 1996. Each exploitation pattern is shown to be a function of the region fished, the catching gear employed, and the growth and migrations of the cuttlefish population. Some implications for stock management are discussed.

Growth, mortality and recruitment rates of giant clams, Tridacna gigas and T. derasa, at Michaelmas Reef, central Great Barrier Reef, Australia

Growth, recruitment and mortality rates of a population of giant clams (Tridacna gigas and T. derasa) were monitored between 1978 and 1985 in a 2.7 ha study area on Michaelmas Reef, Great Barrier Reef. The initial 1978 census revealed the presence of 1166 T. gigas and 44 T. derasa. For T. gigas, the Fabens method provided growth-parameter estimates of L∞ = 85.7 cm, K = 0.07 and to = 0.732 year. The generated von Bertalanffy growth curve was a relatively poor fit to an empirical growth curve. A better description of growth was obtained for younger clams by using a forced value of L∞, = 80 cm, yielding K = 0.105 and to = 0.145 year. There was marked variability in the growth rate of individual clams, which has implications for the aquaculture industry. Average annual mortality rates in two census intervals (1978 to 1980-81 and 1980-81 to 1985) were 3.4 and 10.7% respectively. A comparison of the calculated size structure (assuming constant recruitment) with the observed size structure clearly suggested that recruitment was not constant and had declined drastically from a peak in the 1950s. For the much smaller population of T. derasa, the Fabens routine yielded estimates of L∞ = 46.91 cm, K= 0.108 and to = -0.188 year. The average annual mortality rate was 4.4%. Trends in recruitment could not be determined because of the small sample size. This study has highlighted the need for follow-up studies of this and other populations of giant clams if we are to understand more fully the processes of growth, recruitment and mortality in wild stocks and the implications for stock management and aquaculture.

Analysis of 70-Year Catch Statistics for Atlantic Salmon (Salmo salar) in the River Wye and Implications for Management of Stocks

Farmland consolidation is the act of regulating, improving, and comprehensively renovating the structure, quality, and layout of field, water, road, forestry, and village in the countryside in a certain area by means of administration, economy, law, and engineering techniques according to the goal and usage defined by land use planning, so as to improve farmland use rate and its output rate, to increase farmland area, and to achieve better productive, living, and ecological environment. Recently, farmland consolidation has been carried out all over the country, especially in its economy-developed regions. But, unscientific planning and unsuitable farmland consolidation engineering have negative effects on field ecological system. In this paper, based on the technology of GIS and RS, the basic theories and methods of landscape ecology and a compositive grading method were applied to analysis the dynamics of farmland landscape fragmentation in Tongxiang county. The results showed that the farmland landscape fragmentation in this county was strongly affected by consolidation. More attention should be paid to the protection of farmland landscape during consolidation, and to avoid or decrease the negative effects resulted from unscientific planning and unsuitable farmland consolidation engineering.