Declining stocks of Lake Tana’s endemic Barbus species flock (Pisces, Cyprinidae): natural variation or human impact?

The only remaining species flock of endemic, large cyprinid fishes is found in Lake Tana, Ethiopia. A monthly experimental trawl program was conducted in 1991-1993 and 1999-2001, sampling 12 stations distributed over three habitats differing in depth and distance to shore. The aim was to compare the total abundance, spatial distribution and proportion of juveniles of the most common Barbus species in the Bahar Dar Gulf between both periods. We found a sharp reduction (75%) in total abundance, both in number and biomass of the Barbus species and even more (90%) in the number of juveniles between the two periods. However, the spatial distribution of the different Barbus species over the three habitats had not changed. High natural variability in fish stocks might be expected in environmentally unstable lakes. Although strongly pulsed (seasonal), Lake Tana is a relatively stable system. No major differences were found in abiotic parameters in 1990s that could have caused the dramatic changes in abundance. The most likely explanation is the negative impact of the motorised, commercial gillnet fishery targeting the spawning aggregations of these barbs. The drastic decline in juveniles points especially towards serious recruitment over-fishing. The results stress the need for the immediate development of a sound management plan focussing on fishing effort restrictions during the Barbus breeding season.     

Growth and mortality of the catfish, Hemisynodontis membranaceus (Geoffroy St. Hilaire), in the northern arm of Lake Volta, Ghana

Estimates of growth and mortality of the catfish, Hemisynodontis membranaceus (Geoffroy St. Hilaire), in Lake Volta were obtained from length composition data compiled in 1995 and 1996. The von Bertalanffy growth function (VBGF) estimates were: L _∞=44.5 cm standard length; K= 0.62 year⁻¹; and t ₀=−0.23 years. Natural mortality rate, M , was 1.20 year⁻¹. Total mortality rate, Z , was computed as 4.39 year⁻¹ and the exploitation ratio ( E = F / Z ) was 0.72. Although the fish is estimated to have longevity of about 5 years, those exploited are normally less than 2 years of age, which is indicative of growth over-fishing. In order to arrest over-exploitation of the species, there is a need to establish 'lake reserves'. In addition, the fisheries management should be devolved from the state to the local level to compel fishermen to take greater responsibility for the sustainability and conservation of the fisheries.     

Ecological repercussions of historical fish extraction from the Southern Ocean

A major mid-1980s shift in ecological structure of significant portions of the Southern Ocean was partially due to the serial depletion of fish by intensive industrial fishing, rather than solely to climate factors as previously hypothesized. Over a brief period (1969–1973), several finfish stocks were on average reduced to <50%, and finally (mid-1980s) to <20%, of original size. Despite management actions, few stocks have recovered and some are still declining. Most affected species exhibit K-selected life-history patterns, and before exploitation presumably fluctuated in accordance with infrequent strong year classes, as is true of such fish elsewhere. A climate regime, the Southern Annular Mode, once oscillated between two states, but has remained in its 'positive mode' since the time of the fish extraction. This may have increased finfish vulnerability to exploitation. As breeding stocks decreased, we hypothesize that availability of annually produced juvenile fish fed upon by upper-level predators remained low. Correlations between predator populations and fish biomass in predator foraging areas indicate that southern elephant seal Mirounga leonina , Antarctic fur seal Arctocephalus gazella , gentoo penguin Pygoscelis papua , macaroni penguin Eudyptes chrysolphus and 'imperial' shag Phalacrocorax spp. – all feeding extensively on these fish, and monitored at Marion, Crozet, Kerguelen, Heard, South Georgia, South Orkney and South Shetland Islands, where fishing was concentrated – declined simultaneously during the two periods of heavy fishing. These patterns indicate the past importance of demersal fish as prey in Antarctic marine systems, but determining these interactions' ecological mechanisms may now be impossible.     

大西洋蓝鳍金枪鱼资源开发与养护问题分析

长达数十年的持续不断的高强度捕捞使得大西洋蓝鳍金枪鱼资源处于严重衰退状态。通过渔获数据分析大西洋蓝鳍金枪鱼资源开发状况,结合区域渔业管理制度和养护措施,探讨大西洋蓝鳍金枪鱼管理存在的问题与不足。分析认为:各个国家出于本国利益考虑忽视总可捕量的科学建议,使得大西洋金枪鱼养护国际委员会最终确定的总可捕量高于科学家们的建议量;现有分配机制有失公平,偏离国际法公平原则,因而其不能有效引导各国采取积极养护措施;大西洋金枪鱼养护国际委员会对大西洋蓝鳍金枪鱼捕捞活动监管是整个管理过程中脆弱的环节,且因缺乏有效执行机制而未能有效限制非法捕捞。提出改善总可捕量设定过高现状,使总可捕量设定符合使资源达到最大可持续产量的目标;改进配额分配机制,保障大西洋蓝鳍金枪鱼资源利用的公平合理;加强大西洋蓝鳍金枪鱼捕捞活动监管,完善入渔管理机制,推动成员履约。