The biology of Lates species (Nile perch) in Lake Tanganyika, and the status of the pelagic fishery for Lates species and Luciolates stappersii (Blgr.)

Three Lates species, L. mariae (Stdr.), L. microlepis (Blgr.), L. angustifrons (Blgr.) and Luciolates stappersii (Blgr.) which belongs to a monotypic genus occur only in Lake Tanganyika. They are the principal predators in the pelagic and benthic fish communities. Data are presented on distribution, growth, length-weight relationships, condition, reproduction and food in the Lates spp. After a post-larval pelagic phase, each Lates sp, spends 1 year in littoral weed. Thereafter, L. mariae adopts a benthic habitat moving into deep water, L. microlepis becomes exclusively pelagic and L. angustifrons occurs from the littoral to near the limit of the oxygenated zone. Cycles in breeding correspond with seasonal maxima in clupeid prey, and changes in growth rate within a year are probably related to physiological cycles rather than to variations in the physical environment. Salient morphological and ecological adaptations are discussed. Local abundance and vagility of Lates stocks seem related to basin morphometry and depth of the oxygenated layer. L. microlepis has most vagility. The Lates appear highly susceptible to intensive fishing by purse-seines, and have been fished-up in the southeast arm in Zambia and in the Burundi sector, where only small populations remain. With reduced predation, the clupeid biomass has increased. The change to dominance of small species may be accomplished rapidly in L. Tanganyika under such conditions of exploitation. However, the type of species successions well-known in large temperate lakes are unlikely to occur because of the high degree of specialization in these fishes. A simplified pelagic community is foreseen, consisting almost entirely of clupeids and Luciolates, in which the total fish biomass will be determined above some intensive level of fishing and from year to year by the environmental factors controlling survival of young and plank-tonic food supply. Quantitative equilibrium between the few species will be determined mainly by gear selectivity and total fishing stress, and the community may be sufficiently robust by reason of its rapid turn-over rates to permit a largely empirical approach to management.     

Landed and discarded catches from commercial prawn trammel net fishery

This study presents estimates of numbers and biomass of discarded and landed catches from the commercial prawn trammel net fishery in İzmir Bay. A total of 43 finfish and 29 invertebrate species were identified in catches sampled between May and October 2003, with 26 species landed by fishers and 46 species discarded. Melicertus kerathurus, Mullus barbatus and Solea solea dominated the landed catches. Average ratio of landed to discarded fish by number and weight was estimated as 1 : 2.32 and 1 : 0.77, respectively, demonstrating relatively low by‐catch ratios compared to other shrimp by‐catch studies. This study shows that trammel nets have a much lower discard ratio than the shrimp trawls in other parts of the world. The benefits of shifting toward passive trammel nets in coastal areas are discussed.     

Effects of climate‐driven primary production change on marine food webs: implications for fisheries and conservation

Climate change is altering the rate and distribution of primary production in the world's oceans. Primary production is critical to maintaining biodiversity and supporting fishery catches, but predicting the response of populations to primary production change is complicated by predation and competition interactions. We simulated the effects of change in primary production on diverse marine ecosystems across a wide latitudinal range in Australia using the marine food web model Ecosim. We link models of primary production of lower trophic levels (phytoplankton and benthic producers) under climate change with Ecosim to predict changes in fishery catch, fishery value, biomass of animals of conservation interest, and indicators of community composition. Under a plausible climate change scenario, primary production will increase around Australia and generally this benefits fisheries catch and value and leads to increased biomass of threatened marine animals such as turtles and sharks. However, community composition is not strongly affected. Sensitivity analyses indicate overall positive linear responses of functional groups to primary production change. Responses are robust to the ecosystem type and the complexity of the model used. However, model formulations with more complex predation and competition interactions can reverse the expected responses for some species, resulting in catch declines for some fished species and localized declines of turtle and marine mammal populations under primary productivity increases. We conclude that climate‐driven primary production change needs to be considered by marine ecosystem managers and more specifically, that production increases can simultaneously benefit fisheries and conservation. Greater focus on incorporating predation and competition interactions into models will significantly improve the ability to identify species and industries most at risk from climate change.     

Long-term changes in Krill biomass and distribution in the Barents Sea: are the changes mainly related to capelin stock size and temperature conditions?

Krill plays a significant role in the Barents Sea ecosystem, providing energy transport between different trophic levels. The current paper presents the results of a long-term study (1980–2009) based on pelagic trawl catches from August to September. Our investigations show that the krill species were distributed widely in the Barents Sea and that the largest krill concentrations were restricted to the west-central and eastern parts of the Barents Sea. The current paper presents the relative biomass indices, and the estimates must be interpreted as minimum biomass. The mean annual krill biomass was estimated to be 22 million tonnes in wet weight, with the highest values being as much as 48 million tonnes. Capelin is the largest pelagic stock, and in some years, their biomass can amount to 4–7 million tonnes, which can impose high predation pressure on krill. When their biomass is high, capelin may consume close to 26 million tonnes annually. The predation from pelagic (herring and blue whiting) and bottom (cod and haddock) fish species was much lower, being 9 and 1 million tonnes, respectively. A negative relationship between krill biomass and capelin stock size above 74°N was observed during the study period. However, during the last decade, the krill biomass has increased despite heavy predation from capelin in some years. A positive significant linear relationship between the mean annual Kola temperature and the krill biomass seems to indicate that the recent warming conditions have favourable impacts on the krill populations in the Barents Sea.     

Ecological role and historical trends of large pelagic predators in a subtropical marine ecosystem of the South Atlantic

Large pelagic predators occupy high positions in food webs and could control lower trophic level species by direct and indirect ecological interactions. In this study we aimed to test the hypotheses: (1) pelagic predators are keystone species, and their removals could trigger impacts on the food chain; (2) higher landings of pelagic predators could trigger fishing impacts with time leading to a drop in the mean trophic level of catches; and (3) recovery in the pelagic predators populations, especially for sharks, could be achieved with fishing effort reduction. We performed a food web approach using an Ecopath with Ecosim model to represent the Southeastern and Southern Brazil, a subtropical marine ecosystem, in 2001. We then calibrated the baseline model using catch and fishing effort time series from 2001 to 2012. Afterwards, we simulated the impact of fishing effort changes on species and assessed the ecological impacts on the pelagic community from 2012 to 2025. Results showed that the model was well fitted to landing data for the majority of groups. The pelagic predators species were classified as keystone species impacting mainly on pelagic community. The ecosystem was resilient and fisheries seem sustainable at that time. However, the temporal simulation, from 2001 to 2012, revealed declines in the biomass of three sharks, tuna and billfish groups. It was possible observe declines in the mean trophic level of the catch and in the mean total length of landings. Longline fisheries particularly affected the sharks, billfish and swordfish, while hammerhead sharks were mostly impacted by gillnet fishery. Model simulations showed that large sharks’ biomasses could be recovered or maintained only after strong fishing effort reduction.     

Approaches to estimating fish biomass and potential yield in Lake Tanganyika

The annual fish yield of 73 000 tonnes (22.5 kg/ha) from Lake Tanganyika consists primarily of two planktivorous clupeids Stolothrissa tanganicae (Stdr.) and Limnothrissa miodon (Blgr.) and their centropomid predators, three Lates species (Nile perch) and Luciolates stappersii (Blgr.). At the north and south ends of the lake, the Lates spp. have been fished-up by purse-seines, and the clupeids and young L. stappersii predominate there. Estimates of pelagic fish biomass may employ (i) catch and fishing effort data (ii) abiotic variables (iii) echo-sounding surveys (iv) tropho-dynamic relationships.     

The Ecuadorian Artisanal Fishery for Large Pelagics: Species Composition and Spatio-Temporal Dynamics

The artisanal fisheries of Ecuador operate within one of the most dynamic and productive marine ecosystems of the world. This study investigates the catch composition of the Ecuadorian artisanal fishery for large pelagic fishes, including aspects of its spatio-temporal dynamics. The analyses of this study are based on the most extensive dataset available to date for this fishery: a total of 106,963 trip-landing inspection records collected at its five principal ports during 2008 ‒ 2012. Ecuadorian artisanal fisheries remove a substantial amount of biomass from the upper trophic-level predatory fish community of the eastern tropical Pacific Ocean. It is estimated that at least 135 thousand metric tons (mt) (about 15.5 million fish) were landed in the five principal ports during the study period. The great novelty of Ecuadorian artisanal fisheries is the “oceanic-artisanal” fleet component, which consists of mother-ship (nodriza) boats with their towed fiber-glass skiffs (fibras) operating with pelagic longlines. This fleet has fully expanded into oceanic waters as far offshore as 100°W, west of the Galapagos Archipelago. It is estimated that nodriza operations produce as much as 80% of the total catches of the artisanal fishery. The remainder is produced by independent fibras operating in inshore waters with pelagic longlines and/or surface gillnets. A multivariate regression tree analysis was used to investigate spatio-environmental effects on the nodriza fleet (n = 6,821 trips). The catch species composition of the nodriza fleet is strongly influenced by the northwesterly circulation of the Humboldt Current along the coast of Peru and its associated cold waters masses. The target species and longline gear-type used by nodrizas change seasonally with the incursion of cool waters (< 25°C) from the south and offshore. During this season, dolphinfish (Coryphaena hippurus) dominates the catches. However, in warmer waters, the fishery changes to tuna-billfish-shark longline gear and the catch composition becomes much more diverse.     

Assessing the risk of alternative management strategies in a Mediterranean fishery: protecting the younger vs reducing fishing effort

A stochastic age-structured population model was developed to explore biologically favourable levels of effort and closing periods within the sardine pelagic fishery in the eastern Mediterranean Sea. Results suggested that the developed age-structured model captured the observed biomass fluctuations and catches reasonably well and represents the first comprehensive investigation of alternative management strategies for eastern Mediterranean sardine fishery that include stochasticity. The present study provided direct evidence for the importance of the correct timing of the temporal fishing ban. Significant benefits were found both in terms of biomass and catch from a corrective shift in the fishing closed period. The current findings suggested that protecting the younger age groups from fishing in the period October–December, by shifting the ban period earlier than December may profit, biologically, the stock and economically the fishing sector. Progressive reductions in fishing mortality/effort also yield significant positive biological and fishery benefits in the short term.     

Pelagic cephalopods from eastern Australia: species composition, horizontal and vertical distribution determined from the diets of pelagic fishes

The distribution and relative biomass of cephalopods from pelagic waters off eastern Australia was examined between 1997 and 2004 from stomach contents of swordfish, yellowfin tuna and dolphinfish taken in the domestic longline fishery. A total of 38 taxa from 19 families were identified. Comparison of the species composition of the three predators indicated pronounced differences in cephalopod species composition. In swordfish, species of the family Ommastrephidae, particularly Ommastrephes bartramii (Lesueur 1821) and Nototodarus gouldi (McCoy 1888) dominated, whereas a more diverse mix of species was identified from yellowfin-sampled cephalopods. Todaropsis eblanae (Ball 1841) was the main cephalopod sampled from the surface-dwelling dolphinfish. For swordfish-sampled cephalopods, significant relationships were found between biomass and season, fluorescence and year. In yellowfin tuna, cephalopod biomass was significantly correlated with season, area and sea surface temperature. Significant factors differed between predator-sampler, possibly reflecting the limits of the predator, but could also give insights into individual cephalopod species distributions. However, the increase in cephalopod biomass over summer in both swordfish and yellowfin tuna suggested cephalopod biomass was higher over summer in the region.     

The vertical distribution of fish in the open water area of a deep temperate mesotrophic lake assessed by hydroacoustics and midwater trawling

The fish stock of a deep temperate, mesotrophic lake was sampled at different depths using a fixed‐frame fry trawl, during two nights in mid‐September 2009. Additionally, horizontal and vertical hydroacoustics were used simultaneously to evaluate fish abundance and biomass estimates obtained by the trawl. Roach Rutilus rutilus and smelt Osmerus eperlanus were the dominant species of young‐of‐the‐year (YOY) fish in the trawl catches from the surface layers (0–9 m). Bleak Alburnus alburnus dominated the catch of older fish in the upper part of the surface profile (0–6 m). Around the thermocline (9–13 m) smelt dominated the catches of both the YOY and older fish. Beneath the thermocline (13–36 m) vendace Coregonus albula dominated the catch of YOY fish, and smelt was the only species of older fish in the trawl catches. Species composition, abundance and biomass of the YOY and older fish were heterogeneous throughout the depth profiles of the lake, but only abundance differed significantly between the layers. The hydroacoustics gave relatively similar estimates of abundance and biomass to those obtained by the trawl in all the depths sampled. Our results indicate that there is a clear separation of small fish of different species along the vertical profile of a deep temperate lake during the night, and an unequal vertical distribution of fish abundance and biomass. The similarity of the trawl and hydroacoustics estimates of abundances and biomass indicated that the trawl sampling did not cause important avoidance reactions of small fish during the night in this deep temperate lake (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Interactions between northern shrimp, Pandalus borealis (Pandalidae), and its key predators within the eastern Newfoundland and Labrador marine ecosystem

An examination of population trends for northern shrimp (Pandalus borealis) and its key predators off eastern Newfoundland and Labrador provided the basis for investigating predator-prey relationships within an ecosystem that experienced major changes in species composition during the 1980s and 1990s. Populations of several demersal fish species, known to feed on northern shrimp, declined to historically low levels by the early 1990s and remained depressed thereafter. Some declines were precipitous from the late 1980s to early 1990s, coincident with an increase in shrimp. Populations of other important predator species increased throughout the 1990s along with shrimp. Lacking representative estimates of shrimp consumption, the net effect on predation mortality was unquantifiable and it was not possible to demonstrate with certainty that the major increase in shrimp biomass that occurred throughout the 1990s resulted from a concomitant reduction in predation mortality. Factors (and their interactions) relevant to the dynamics of shrimp populations include the effects of ocean climate, predation, competition, as well as commercial harvesting.     

长江上游三层流刺网渔业现状的调查

1997-1999年对长江上游宜宾江段和合江江段的三层流刺网渔业现状进行了调查。合江江段三层流刺网内网目在10-180mm之间,宜宾江段以50mm以下的网目为主。采集到的鱼类有59种,包括17种长江上游特有鱼类;渔获物组成存在区域、季节和年际之间的差异。目前长江上游的捕捞强度比70年代显著提高。     

一座热带浅水水库敞水区浮游甲壳动物群落结构

为了解热带中小型浅水型水库枯水期浮游动物群落组成与结构稳定性,于2006至2007的三个枯水期,对位于我国雷州半岛甘村水库敞水区进行了每周一次的高频采样,主要以个体大小作为变量分析了浮游甲壳类动物的群落结构的组成.三次枯水期采样中,敞水区中只检测到枝角类4种和桡足类4种.浮游甲壳类的体长范围多集中在400～700μm之间.枝角类的优势种为模糊秀体溞平均体长为533μm,桡足类成体优势种台湾温剑水蚤的平均体长为787μm,无节幼体和剑水蚤桡足幼体丰度与生物量也相对较高.水库浮游甲壳类种类数和丰度的峰值均出现在体长为300～400μm和400-500μm两个等级内.浮游甲壳类的种类和数量均是以小个体为主,除了营养水平还与水体中的捕食压力紧密相关,鱼类的捕食对于浮游甲壳类群落结构的形成作用显著.     

长江口及其邻近水域大型水母资源量动态变化对渔业资源结构的影响

根据2007年5月、2008年5月和6月对长江口及其邻近水域渔业底拖网调查资料,分析了该水域大型水母资源量变化对渔业种类组成、分布和资源量的影响.结果表明:2007年5月-2008年6月,大型水母的平均网获量和最高网获量增加,2008年6月平均网获量为222.2 kg·h-1,最高网获量达到1800 kg·h-1.大型水母主要分布在50 m等深线附近,及100m等深线以浅的东海沿岸流、长江径流和台湾暖流的锋面上,6月大型水母的集中分布范围与5月相比向北偏移,在2007年5月和2008年5月两次调查中均存在一个水母集中分布点(122.5° E,28.5° N).大型水母和渔业资源呈交错分布,大型水母集中分布的区域,渔业资源网获量降低,种类组成也发生了显著变化.在水母网获量高的站位主要以小型种类(竹筴鱼、龙头鱼、发光鲷等)或中上层种类(刺鲳、长蛸)及带鱼为主,刺鲳在2008年6月的调查中占整个渔获量的23.7％,小黄鱼也占有较高的比重,但一些底层种类如黄鮟鏮等在水母集中分布区没有发现,虾蟹类也相对较少.大型水母对长江口渔业生态系统的群落结构和资源量都产生了一定的影响,而且水母可以通过营养级联反应改变系统的能流方式.因此,在保护渔业资源和制定相关的渔业管理措施时,建议将水母作为一个独立的生态类群来考虑.     

Functional and aggregative response of North Sea whiting

The functional response of whiting (Merlangius merlangus L.) to clupeid and gadoid prey was determined from estimates of food intake and prey density at five locations in the North Sea. The intake of most prey types was well described by a type II (decelerating) response, although in some cases a type III (sigmoid) response provided a slightly better fit. Though a saturation level was reached for all types of fish prey, none of the levels corresponded to the maximum digestive capacity of the predator. This was not caused by ingestion of other prey as the amount of other food and fish prey ingested were not negatively correlated. An investigation of the occurrence of fresh fish in the stomachs revealed that fish was ingested almost exclusively during dawn and dusk and the lack of negative correlation between the intake of fish and other prey may thus be a result of the limited time in which fish prey was vulnerable to predation. No aggregative response of the predators was detected towards any of the prey and catches of prey and predators were slightly negatively correlated. There was evidence of an increase in mortality with density at low clupeid densities, but mortality decreased to virtually zero at high densities. Whiting seem therefore unlikely to impose a regulatory effect on their fish prey outside a narrow range of prey densities.     

Shark bycatch and depredation in the U.S. Atlantic pelagic longline fishery

The non-target bycatch of sharks in pelagic longline (PLL) fisheries represents a potential source of compromise to shark populations worldwide. Moreover, shark bycatch and depredation (damage inflicted on gear, bait, and catch) complicates management of sharks and other species, and can undermine the operations and financial interests of the pelagic longline industry. Thus, deducing means to reduce shark interactions is in the best interest of multiple stakeholder groups. Prior to doing so, however, the extent, cause and effect of these interactions must be better understood. In this review we address or conduct the following in relation to the U.S. Atlantic, Gulf of Mexico and Caribbean PLL fishery: (1) U.S. management governing shark interactions in the Atlantic; (2) the primary species encountered and historical shark catch data associated with PLL fishing in the Atlantic; (3) a historical comparison of area-specific shark species catch records between the two primary sources of shark catch data in this fishery; (4) the conditions and dynamics that dictate shark interactions in this fishery, and potential means to reduce these interactions, and; (5) a synthesis of the estimated impacts of this fishery on shark populations relative to other fisheries in the Atlantic. As has been found in other PLL fisheries, the blue shark (Prionace glauca) is clearly the shark species most commonly encountered in this fishery in the Atlantic, and receives the majority of attention in this review. U.S. management areas with high relative shark species diversities had a greater divergence in historical shark species percent-compositions between data sources (Pelagic Observer Program versus mandatory pelagic Logbook databases); this complicates the ability to conclude which species are most impacted by PLL fishing in those areas. The current fishing effort by the U.S. PLL fleet is small compared to that of PLL fishing targeting sharks in the Atlantic by non-U.S. fleets, and therefore poses a comparatively lower threat to the stability of Atlantic shark populations. However, incidental shark encounters are inevitable in U.S. Atlantic PLL fishing operations. Thus, it is in the best interest of all stakeholders in the Atlantic to better understand the extent and conditions governing these interactions, and to explore methods to reduce both their occurrence and those aspects leading to higher rates of incidental shark mortality.     

Effects of brown trout (Salmo trutta L.) stocking and catch‐release practice on angling catches in the River South Rena in southeast Norway

Preserving of fish species and populations is important whether it is for exploitation or just for conservation. Management of fisheries aim to maintain fishable stocks that are attractive to anglers, and different means are performed. In this study from the River South Rena in southeastern Norway, conducted during 1991–2005, the effects of supportive stocking of hatchery reared brown trout (Salmo trutta L.) from 1996, and bag limit (BL) and catch‐release (CR) practice for the target species brown trout, from 2002, were explored. Effects of supplemental brown trout stocking was not noticeable, except from one year following a year of exceptional high number of stocked fish, actually 41% of the catches, whereas in the following years this proportion remained constant about 10%, and the catches remained high in 2003 and 2004, mainly due to increased angling success rate after BL‐CR introduction.     

Top-down control in pelagic systems: a role for invertebrate predation

Limnologists have long recognized the importance of predation in freshwater communities. The majority of study of predator effects has involved vertebrate predators, with emphasis on planktivorous fish. Documented effects of planktivorous fish have been so dramatic that manipulations of their populations are seen by many as potential tools in lake management. However, the success of such manipulations is often less than desired due to the ubiquitous complexity of food webs and the pervasiveness of compensatory responses to food web manipulation. Recently, enormous effort has been applied to the Lake Kinneret pelagic food web in effort to reduced the abundance of the planktivorous Kinneret bleak Acanthobrama terraesanctae and thereby increase the biomass of herbivorous zooplankton in the hopes of increasing water clarity. We compared potential predation pressure on Lake Kinneret herbivorous zooplankton by bleak and the other major zooplankton predators in the lake, the cyclopoid copepods Mesocyclops ogunnus and Thermocyclops dybowskii. We found that, despite having much lower biomass, cyclopoid copepods accounted for a greater portion of the predation mortality on herbivorous zooplankton than bleak. Our results suggest that reductions in predation pressure by bleak will not yield subsequent increases in herbivorous zooplankton biomass. Rather, reductions in bleak predation pressure may allow for increases in cyclopoid copepod abundance and thereby a net increase in predation pressure on herbivorous zooplankton.     

Fishery-Induced Changes in the Subtropical Pacific Pelagic Ecosystem Size Structure: Observations and Theory

We analyzed a 16-year (1996–2011) time series of catch and effort data for 23 species with mean weights ranging from 0.8 kg to 224 kg, recorded by observers in the Hawaii-based deep-set longline fishery. Over this time period, domestic fishing effort, as numbers of hooks set in the core Hawaii-based fishing ground, has increased fourfold. The standardized aggregated annual catch rate for 9 small (<15 kg) species increased about 25% while for 14 large species (>15 kg) it decreased about 50% over the 16-year period. A size-based ecosystem model for the subtropical Pacific captures this pattern well as a response to increased fishing effort. Further, the model projects a decline in the abundance of fishes larger than 15 kg results in an increase in abundance of animals from 0.1 to 15 kg but with minimal subsequent cascade to sizes smaller than 0.1 kg. These results suggest that size-based predation plays a key role in structuring the subtropical ecosystem. These changes in ecosystem size structure show up in the fishery in various ways. The non-commercial species lancetfish (mean weight 7 kg) has now surpassed the target species, bigeye tuna, as the species with the highest annual catch rate. Based on the increase in snake mackerel (mean weight 0.8 kg) and lancetfish catches, the discards in the fishery are estimated to have increased from 30 to 40% of the total catch.     

Genetic mixed‐stock analysis of lake‐run brown trout Salmo trutta fishery catches in the Inari Basin,northern Finland: implications for conservation and management

Genetic mixed‐stock analysis (MSA) of wild lake‐run brown trout Salmo trutta fishery catches (n = 665) from the Inari Basin (northern Finland) between 2006 and 2008 was carried out using a previously characterized baseline with 30 populations (n = 813) and 13 microsatellite loci. Altogether, 12 populations contributed significantly to mixed‐stock fisheries, with the Ivalojoki system being the major contributor (70%) to the total catch. When catches were analysed regionally, geographically nearby populations were the main contributors to the local catches, indicating that a large proportion of S. trutta occupy lacustrine areas near the natal river mouth rather than dispersing throughout the lake. Similarly, far upstream populations contributed insignificantly to catches. These findings have important implications for the conservation and sustainable fishery management of the Inari system.