Occurrence of gyrodactylids on wild Atlantic salmon, Salmo salar L., in Danish rivers

Three hundred and one parr of wild salmon, Salmo salar L., from three river systems in Denmark were examined in December 2005 and August 2006 for gyrodactylid infections. The species determination was done by morphological analyses of the opisthaptoral hard parts and molecular analyses of the internal transcribed spacer (ITS) region using PCR restriction fragment length polymorphism (RFLP) and sequencing. Five species of the genus Gyrodactylus were found: Gyrodactylus derjavini, Gyrodactylus teuchis, Gyrodactylus truttae, Gyrodactylus arcuatus and Gyrodactylus salaris. The parasite species G. derjavini dominated over G. teuchis and G. truttae and only one specimen of G. arcuatus and one specimen of G. salaris were recovered. This specimen of G. salaris had the same mutation in the ITS-region as a previously described non-pathogenic Danish G. salaris variant. The prevalence of gyrodactylids on the salmon ranged from 0% to 100% but the mean abundance never exceeded 12.8.     

Identification of Gyrodactylus (Monogenea) species parasitizing salmonid fish using DNA probes

Abstract. Oligonucleolides GsV4B, GdV4 and GtV4, complementary to sequences from Gyrodactylus Salaris, G. derjavini and G. truttae , respectively, were designed following examination of the sequence of the small subunit ribosomal RNA (srRNA) gene from each species. The oligonucleotides were end-labelled with digoxigenin for use us species-specific probes. The V4 region of the srRNA gene was amplified from individual gyrodactylids using the polymerase chain reaction (PCR) and aliquots of the PCR products werespotted onto nylon membrane. Optimum conditions for hybridization of the oligonucleotide probes to these samples were determined. The use of these three probes against aliquots of PCR product provides an objective, sensitive and specific method suitable for routine diagnostic use in identifying Gyrodactylus species found on salmonid fish, but cannot be used to distinguish G. salaris and G. thymulli .     

Migration of Atlantic salmon, Salmo salar L., parr through a Norwegian fjord: potential infection path of Gyrodactylus salaris

Abstract. Hatchery-reared Atlantic salmon, Salmo salar L., parr (0+) ranging in length from 74 to 95 mm were sampled in a stream with no hatchery in its drainage area. No fish had previously been released in this stream. The parr, therefore, had migrated during a period when the salinity of the fjord was between 20 and 30·5%₀, a minimum distance of 2.7km through the fjord, which was the distance to the nearest hatchery. This is the first observation of pre-smolt migration between streams via a fjord. Earlier, the salmon in the river had been infected by the monogean Gyrodactylus salaris. Probably, this infection had been spread by parr migrating from a neighbouring stream through the fjord. Parasite dispersion coincided with a period of low fjordwater salinity (12—14.5%₀) and sea temperatures ranging from 8.2 to 16.5°C, which is within the suggested tolerance level of G. salaris survival on salmon parr.     

The suitability of anadromous Arctic charr as host and vector of the monogenean Gyrodactylus salaris

Abstract –  In 2000, the Atlantic salmon ( Salmo salar L.) stock in the river Signaldalselva, North Norway was found to be infected with the monogenean Gyrodactylus salaris (Malmberg). This river system has a multispecies fish community of nine species in total. Low densities of Atlantic salmon parr were found during electrofishing in October (2003) with a prevalence of 94% and a mean abundance of 848 G. salaris. This watercourse also holds a riverine stock of anadromous Arctic charr ( Salvelinus alpinus ), which is not common in Norway. Among the parr of Arctic charr, 64% were infested with an abundance of 23 parasites per fish. The most heavily infected 0+ and 1+ juvenile Arctic charr had 469 and 534 parasites, respectively, indicating that the parasites are able to reproduce on Arctic charr in the wild. There was little variation in the rates of infestation of Arctic charr along the studied stretch of the river, even in areas with very low densities of Atlantic salmon parr. The parr of anadromous Arctic charr seem to be suitable as long-term hosts for G. salaris . Moreover, a few (22%) infested adult sea-running Arctic charr were captured after ascending the river in the autumn, which shows that large fish may also act as carriers of G. salaris . However, no parasites were recorded on potential anadromous Arctic charr before descending during early spring (April 2003 and April 2004). Thus, it is still not clear whether anadromous Arctic charr are able to disperse the parasite between watercourses. So far, it is uncertain whether G. salaris may increase the mortality rates of Arctic charr and thereby be a threat to these unusual occurring riverine anadromous Arctic charr stocks.     

Epidemiology of Gyrodactylus salaris (Monogenea) in the River Tornionjoki, a Baltic wild salmon river

The occurrence of Gyrodactylus salaris in the River Tornionjoki was investigated in 2000-2004. Infection of salmon parr, Salmo salar, was common in the uppermost reach of the river system but decreased downstream and was rare in the lowermost reach. This pattern was consistent across the study period regardless of varying water temperatures. The oldest age groups of parr were more often infected than younger ones throughout the river system, irrespective of their origin (wild or stocked). Parasite-free hatchery-reared 1-year-old parr became infected during their first summer in the wild. Downmigrating salmon smolts had a high prevalence of infection, but their role in the distribution of infection seemed unimportant. On grayling, Thymallus thymallus, we observed only the grayling-specific clade of Gyrodactylus. We found no indication of grayling participating in the epidemiology of infection on salmon. The salmon parr and smolt population in the Tornionjoki has been at its height during the late 1990s and 2000s. Our results indicate that G. salaris infection in this Baltic river has no devastating effects on the salmon population as it has had in salmon rivers flowing into the North Atlantic and White Sea.     

额尔齐斯河三代虫种类研究及新纪录种记述

为探究额尔齐斯河水域中鱼类寄生三代虫的种类以及感染情况,给新疆水产业的发展及我国三代虫的研究提供基础资料,2009年8月至2014年8月选取额尔齐斯河的北屯镇、185团场、布尔津和哈巴河4个采样点,自捕或者从当地渔民处购买材料鱼,通过染色观察三代虫的形态特征,对其几丁质结构进行绘图、测量及比较,同时对不同种类三代虫的感染率、感染强度、感染丰度以及宿主专一性进行分析。结果表明,4个采样点共发现三代虫14种,其中丁 三代虫（Gyrodactylus tinca Malmberg,1957）、白斑狗鱼三代虫（Gyrodoctylus lucii Kulakowskaja,1951）、梭鲈三代虫（Gyrodoctylu luciopercae Gusev,1962）、马氏三代虫（Gyrodactylus markakulensis Gvosdev,1950）均是国内首次发现,为新纪录种;感染率最高的为鲤（Cyprinus carpio）（23．94％）,最低的为河鲈（Perca fluviatilis）（0．74％）,除东方欧鳊（Abramis brama orientalis）、散鳞镜鲤（Cyprinus carpio haematopterrus）和鲤外,其余的感染率均不超过10％;银鲫（Carassius auratus gibelio）、散鳞镜鲤和鲤为易感宿主,方均比（S2／X）都大于1,呈聚集分布;其余的小于1,呈均匀分布。三代虫宿主专一性较强,但有少数种类三代虫寄生2～4种宿主的情况,也存在同一宿主感染2～3种三代虫的现象,与单殖吸虫的宿主专一性基本一致。     

Introduction and establishment of Gyrodactylus salaris Malmberg, 1957, on Atlantic salmon, Salmo salar L., fry and parr in the River Vefsna, northern Norway

Abstract. Gyrodactylus salaris was probably introduced into the River Vefsna, a large salmon river in northern Norway, by stocking of Atlantic salmon smolts from infected hatcheries in 1975 and 1977. An outbreak of G. salaris among salmon parr was observed in 1978. During the following 2 years, the parasite spread throughout the entire watercourse. There was a rapid rise in prevalence rate and degree of infection. The density of salmon parr decreased from a high level before the Gyrodacrylus outbreak to close to zero after the outbreak. Salmon parr mortality is probably associated with an increase in the parasite burden. Infected fish appear to survive for slightly more than 1 year and this time period is sufficient for transmission of the parasite to the next year class. The total catch of salmon ascending the river has also greatly decreased in recent years.     

Transmission dynamics of the monogenean Gyrodactylus salaris under seminatural conditions

Tracking individual variation in the dynamics of parasite infections in wild populations is often complicated by lack of knowledge of the epidemiological history of hosts. Whereas the dynamics and development of Gyrodactylus salaris Malmberg, 1957, on Atlantic salmon, Salmo salar L., are known from laboratory studies, knowledge about infection development on individual wild fishes is currently sparse. In this study, the dynamics of an infection of G. salaris on individually marked Atlantic salmon parr was followed in a section of a natural stream. During the 6‐week experiment, the prevalence increased from 3.3 to 60.0%, with an average increase in intensity of 4.1% day^?1. Survival analyses showed an initially high probability (93.6%) of staying uninfected by G. salaris, decreasing significantly to 37% after 6 weeks. The results showed that even at subarctic water temperatures and with an initially low risk of infection, the parasite spread rapidly in the Atlantic salmon population, with the capacity to reach 100% prevalence within a short summer season. The study thus track individual infection trajectories of Atlantic salmon living under near‐natural conditions, providing an integration of key population parameters from controlled experiments with the dynamics of the epizootic observed in free‐living living populations.     

The placement of Gyrodactylus salmonis (Yin & Sproston) in the molecular phylogeny of studied members of the Gyrodactylus wageneri‐group parasitizing salmonids

The molecular phylogeny of Gyrodactylus salmonis from brook charr, Salvelinus fontinalis, rainbow trout, Oncorhynchus mykiss, cutthroat trout, O. clarkii, and Atlantic salmon, Salmo salar, in Canada is presented using sequences from ITS‐rDNA and the mitochondrial COX 1 gene. Sequence variation among G. salmonis specimens from the different North American hosts was consistent with within‐species variation reported for other Gyrodactylus. Sequence data are compared to those from other members of the wageneri group parasitizing salmoniform fishes in northern Europe (G. derjavini, G. derjavinoides, G. lavareti, G. salaris, G. salvelini, G. teuchis and G. truttae) and Asia (G. brachymystacis). Sequence divergence between G. salmonis and the recently described G. salvelini on Arctic charr, Salvelinus alpinus, in Finland was consistent with within‐species levels of variation in Gyrodactylus; however, phylogenetic analyses and morphological comparisons provided evidence of their distinctiveness such that they appear to be sister species. They grouped with G. lavareti (a parasite of a coregonid) to form a clade separate from European and Asian species of the wageneri lineage known from salmonid fish. Further study of gyrodactylids from across salmonid, coregonid and thymallid fish in the northern hemisphere would shed more light on the phylogeography of these parasites and serves as an important backdrop to understanding the evolution of their emergent virulence.     

Longline hook selectivity for red tilefish Branchiostegus japonicus in the East China Sea

The catch of red tilefish by the Japanese longline fishery in the East China Sea has steeply declined during the last two decades, and resource management is clearly required. This study presents the size selectivity of hooks for red tilefish as a method to control first capture size, based on the results of longline fishing experiments using “Tainawa-bari” hooks of several sizes: Japanese nominal sizes J#10, J#11, J#12, J#13, J#14 and J#16. On the assumption of geometrical similarity in the combination of upper jaw length u and hook width g at a given retention probability, a single master curve s(R) = exp (−14.42 + 8.85R)/(1 + exp [−14.42 + 8.85R]) was estimated in terms of relative upper jaw length to hook width, R = u/g, using the SELECT method based on a multinomial distribution. The relative upper jaw length for 50% retention is 1.63, and the selection range is 0.25. Almost all fish over 300 mm in total length were retained by a hook of size J#12, but smaller fish with less commercial value were also caught. Moreover, J#10 hooks catch fish with total lengths of >250 mm efficiently. This indicates that J#10 and J#12 hooks that are currently used are too small to optimize yield per recruit and also for resource conservation.     

Microhabitat selection of Gyrodactylus salaris Malmberg on different salmonids

The microhabitat selection of the ectoparasite Gyrodactylus salaris (Lærdalselva strain, Norway) was investigated concurrently with studies on the parasite population growth on five strains of Atlantic salmon, Salmo salar L., and a strain of Danish rainbow trout, Oncorhynchus mykiss (Walbaum). The salmon used were hatchery‐reared parr of East Atlantic strains [River Conon (Scotland), River Storå (western Denmark) and River Ätran (western Sweden)] and Baltic strains [Lule and Ume (eastern Sweden)]. The location and numbers of parasites were recorded on anaesthetized fish once a week from week 0 to week 8. The mean abundance of G. salaris steadily increased to high levels on the River Conon, Storå and Ätran strains until the end of the experiment. The mean abundance of G. salaris on the two Baltic strains (River Lule älv and River Ume älv) initially increased but after 4–7 weeks the growth of the parasite infrapopulations decreased markedly. The Danish rainbow trout strain showed the lowest abundances of all the fish species and strains. Gyrodactylus salaris preferentially selected the fins and head region when colonising the hosts (all species and strains). Increasing percentages of G. salaris on the tail fins of the East Atlantic strains and rainbow trout were found during the course of infection, whereas the two Baltic salmon strains experienced a decreasing percentage of parasites in this microhabitat.     

Effect of changes in hook pattern and size on catch rate, hooking location, injury and bleeding for a number of tropical reef fish species

The Queensland Great Barrier Reef line fishery in Australia is regulated via a range of input and output controls including minimum size limits, daily catch limits and commercial catch quotas. As a result of these measures a substantial proportion of the catch is released or discarded. The fate of these released fish is uncertain, but hook-related mortality can potentially be decreased by using hooks that reduce the rates of injury, bleeding and deep hooking. There is also the potential to reduce the capture of non-target species though gear selectivity. A total of 1053 individual fish representing five target species and three non-target species were caught using six hook types including three hook patterns (non-offset circle, J and offset circle), each in two sizes (small 4/0 or 5/0 and large 8/0). Catch rates for each of the hook patterns and sizes varied between species with no consistent results for target or non-target species. When data for all of the fish species were aggregated there was a trend for larger hooks, J hooks and offset circle hooks to cause a greater number of injuries. Using larger hooks was more likely to result in bleeding, although this trend was not statistically significant. Larger hooks were also more likely to foul-hook fish or hook fish in the eye. There was a reduction in the rates of injuries and bleeding for both target and non-target species when using the smaller hook sizes. For a number of species included in our study the incidence of deep hooking decreased when using non-offset circle hooks, however, these results were not consistent for all species. Our results highlight the variability in hook performance across a range of tropical demersal finfish species. The most obvious conservation benefits for both target and non-target species arise from using smaller sized hooks and non-offset circle hooks. Fishers should be encouraged to use these hook configurations to reduce the potential for post-release mortality of released fish.     

Are circle hooks effective in reducing incidental captures of loggerhead sea turtles in a Mediterranean longline fishery?

• 1. A known fishing hot spot for loggerhead sea turtles (Caretta caretta) in the Mediterranean Sea is in the waters of the Strait of Sicily where interactions with fish hooks and branchlines are believed to be a major cause of mortality for sea turtles.
• 2. Hooks with different shapes but a similar gape width (circle hook size 16/0 vs J hook size 2) were tested in order to determine the potential effectiveness of the hook design to both reduce sea turtle capture as well as to maintain acceptable levels of target species capture rates in a shallow‐set longline swordfish fishery in the Mediterranean.
• 3. Seven experimental fishing trips, 30 000 hooks total, were conducted on a single commercial fishing vessel (18 m in length) in the Strait of Sicily during the months of July through October over a period of three years from 2005 to 2007. Circle and J hooks were alternated along the mainline.
• 4. A total of 26 sea turtles were hooked, all immature‐size Caretta caretta. Turtles were caught at a statistically greater frequency on J hooks than on circle hooks. The capture rate, weight, and upper jaw fork length of the target species were not significantly different between the two types of hooks employed.
• 5. Five sea turtles swallowed the hook and in all such cases these were J type. Circle hooks tended to be located externally and were more easily detected by fishermen, and could be removed with the correct dehooking action before returning the turtle to the sea.
• 6. These findings suggest that 16/0 circle hooks can effectively reduce the incidental capture of immature loggerhead sea turtles in a Mediterranean swordfish longline fishery without affecting the catch size of the target species.

Copyright © 2009 John Wiley & Sons, Ltd.     

Deep hooking,landing success and gear loss using inline and offset circle and J hooks when bait fishing for white sturgeon

The issue of deep hooking is of concern in white sturgeon, Acipenser transmontanus Richardson, fisheries because nearly all anglers use bait with a stationary presentation on the river bottom to catch them, and bait fishing is often associated with higher instances of deep hooking and hooking mortality. Deep hooking rates, landing success and catch rates were investigated for anglers bait fishing for white sturgeon using circle and J hooks with inline and offset alignments fished with both active and passive hook‐setting methods. Anglers hooked 578 white sturgeon and landed 508 fish, ranging in size from 60 to 316 cm total length (mean = 137 cm). Deep hooking rates averaged 0.6% and did not differ between hook types, hook alignments or hook‐setting methods. Landing success (the proportion of hooked sturgeon that were successfully landed) and catch rates were also equivalent between hook types, hook alignments and hook‐setting methods; landing success averaged 88% and catch rates averaged 0.27 fish/hr. Results of this study indicate that deep hooking is rare when angling for white sturgeon using standard bait‐fishing gear regardless of hook‐setting method or whether circle or J hooks were used; regulations restricting hook type in sturgeon bait fisheries are therefore unwarranted.     

Are circle hooks an effective tool for conserving marine and freshwater recreational catch‐and‐release fisheries?

• 1. Circle hooks have rapidly become popular among recreational anglers, based largely on the assumption that their use aids in the conservation of fisheries resources by reducing gut hooking, and hence mortality. In addition, circle hooks are intended to facilitate jaw hooking. Unfortunately, these assumptions have been perpetuated by anecdotal reports with very little rigorous scientific information to support these assertions.
• 2. A number of recently published, forthcoming, and grey literature reports provide an opportunity to review briefly and synthesize research conducted on circle hooks. We surveyed literature databases and also used questionnaires to solicit information from unpublished or in‐progress circle hook research.
• 3. Although among studies the results have been quite disparate, overall the mortality rates were consistently lower for circle hooks than J‐style hooks. In addition, circle hooks were more frequently hooked in the jaw, and less frequently hooked in the gut than conventional hook types. There is no doubt that in some marine fisheries, such as tuna, billfish, and striped bass, capture efficiency remains high and injury and mortality rates are drastically reduced. However, in other species (e.g. bluegill), injury can actually be more severe from circle hooks relative to some other hook types. In other species, such as largemouth bass, circle hooks have minimal conservation benefit, but have reduced capture efficiency relative to conventional hook designs.
• 4. Factors such as hook size, fishing style, fish feeding mode, and mouth morphology all appear to affect the effectiveness of circle hooks. For these reasons, it is difficult to promote the adoption of the use of circle hooks as a panacea for all fish and fisheries. Instead, we recommend that management agencies focus on recommending circle hooks only for instances for which appropriate scientific data exist.
• 5. The recent interest in circle hooks has been beneficial for stimulating interest and research on the role of hook designs in reducing hooking‐related injury and mortality. We encourage tackle manufacturers to continue to develop new hook designs that have the potential to provide conservation benefit to caught and released fish. This paper provides direction to management agencies and outdoor media for disseminating responsible information to anglers regarding the application of circle hooks for conserving fisheries resources.

Copyright © 2004 John Wiley & Sons, Ltd.     

A cross‐taxa assessment of pelagic longline by‐catch mitigation measures: conflicts and mutual benefits to elasmobranchs

Elasmobranch mortality in pelagic longline fisheries poses a risk to some populations, alters the distribution of abundance between sympatric competitors, changing ecosystem structure, processes and stability. Individual and synergistic effects on elasmobranch catch and survival from pelagic longline gear factors, including methods prescribed to mitigate bycatch of other vulnerable taxa, were determined. Overall relative risk of higher circle vs. J‐shaped hook shark catch rates conditioned on potentially informative moderators, from 30 studies, was estimated using an inverse‐precision weighted mixed‐effects meta‐regression modelling approach. Sharks had a 1.20 times (95% CI: 1.03–1.39) significantly higher pooled relative risk of capture on circle hooks, with two significant moderators. The pooled relative risk estimate of ray circle hook catch from 15 studies was not significant (RR = 1.22, 95% CI: 0.89–1.66) with no significant moderators. From a literature review, wire leaders had higher shark catch and haulback mortality than monofilament. Interacting effects of hook, bait and leader affect shark catch rates: hook shape and width and bait type determine hooking position and ability to sever monofilament leaders. Circle hooks increased elasmobranch catch, but reduced haulback mortality and deep hooking relative to J‐shaped hooks of the same or narrower width. Using fish vs. squid for bait increased shark catch and deep hooking. Pelagic stingray (Pteroplatytrygon violacea) catch and mortality were lower on wider hooks. Using circle instead of J‐shaped hooks and fish instead of squid for bait, while benefitting sea turtles, odontocetes and possibly seabirds, exacerbates elasmobranch catch and injury, therefore warranting fishery‐specific assessments to determine relative risks.     

The influence of hook size, type, and location on hook retention and survival of angled bonefish (Albula vulpes)

Bonefish (Albula spp.) support a circum-tropical sport fishery in which anglers predominantly release angled fish. The influence of hook location, size, and type on hook retention, post-release feeding ability and survival were evaluated. Overall, 46% of bonefish held in large holding tanks expelled hooks within a 14-day observation period. Hooks located in the lip were expelled 2.6 times more frequently than hooks located in the gut. Barbless hooks were expelled 3.9 times faster when located deep in the oral cavity compared to barbed hooks, but there was no difference in expulsion rates among barbed and barbless hooks in shallow-hooked fish. For the two hook sizes studied, hook size had no impact on hook expulsion rates or duration of hook retention regardless of hook location or type. The presence of a hook had no significant effect on weight change, indicating the presence of a hook did not impede feeding ability. No post-release mortality was observed for bonefish during the short duration of this study. Leaving a difficult to remove hook in a bonefish is recommended to increase the likelihood of post-release survival, especially in cases where the threat of bonefish predators is high.     

Circle hooks, 'J' hooks and drop-back time: a hook performance study of the south Florida recreational live-bait fishery for sailfish, Istiophorus platypterus

Abstract  This study evaluates the performance of two types of non-offset circle hooks (traditional and non-traditional) and a similar-sized 'J' hook commonly used in the south Florida recreational live-bait fishery for Atlantic sailfish, Istiophorus platypterus (Shaw). A total of 766 sailfish were caught off south Florida (Jupiter to Key West, FL, USA) to assess hook performance and drop-back time, which is the interval between the fish's initial strike and exertion of pressure by the fisher to engage the hook. Four drop-back intervals were examined (0–5, 6–10, 11–15 and >15 s), and hook performance was assessed in terms of proportions of successful catch, undesirable hook locations, bleeding events and undesirable release condition associated with physical hook damage and trauma. In terms of hook performance, the traditionally-shaped circle hook had the greatest conservation benefit for survival after release. In addition, this was the only hook type tested that performed well during each drop-back interval for all performance metrics. Conversely, 'J' hooks resulted in higher proportions of undesirable hook locations (as much as twofold), bleeding and fish released in undesirable condition, particularly during long drop-back intervals. Non-traditional circle hooks had performance results intermediate to the other hook types, but also had the worst performance relative to undesirable release condition during the first two drop-back intervals. Choice of hook type and drop-back interval can significantly change hook wounding, and different models of non-offset circle hooks should not be assumed to perform equivalently.     

Influence of terminal rig configuration on the anatomical hooking location of line-caught yellowfin bream, Acanthopagrus australis

Abstract  Experiments were carried out to test the dependence of anatomical hooking location on terminal rig configuration, and identify strategies by which hook ingestion can be minimised to reduce post-release mortality of angled yellowfin bream, Acanthopagrus australis (Günther). Eighty-five anglers targeted yellowfin bream using different designs and sizes of circle and J hooks and recorded technical, environmental and biological data for each landed fish. Considerably fewer fish (16.6 and 8.5%, respectively) ingested hooks than those hooked in the mouth (82.1 and 88.6%), and a greater percentage of J hooks were ingested than circle hooks or a J hook modified with a 15-mm horizontal bar. Irrespective of hook type, the rate of ingestion also declined with increasing hook weight and decreasing fish size. In cage experiments, significantly more hooks baited with natural soft baits (e.g. intestines and cephalopods) were ingested than those baited with artificial baits, crustaceans or teleosts, while in the field fewer hooks were ingested using artificial bait (e.g. bread and pellets) than terrestrial animals, teleosts, crustaceans, teleost or terrestrial animal intestines, cephalopods or polychaetes. Similarly, rigs with floats, short traces or a sinker only on the hook, along with angling in environments with at least some current reduced ingestion. Based on this study, anglers targeting yellowfin bream should be encouraged to use circle or modified J hooks and, regardless of the hook design, the largest hook size possible attached to short traces or a sinker only and hard baits.     

Deep water longline selectivity for black spot seabream (<Emphasis Type="Italic">Pagellus bogaraveo</Emphasis>) in the Strait of Gibraltar

Species and size selectivity of the deep water longline traditionally used in commercial fishing of the black spot seabream (Pagellus bogaraveo) were studied in the Strait of Gibraltar with four sizes of hooks. Black spot seabream contributed up to 88% of the catch by number. Catch and by-catch rates differed for the different hooks and fishing trials. Significant differences in average fish length between all hooks, except in one case, were found. The comparison of two experimental fishing trials within 4 years indicates a displacement towards smaller sizes in the size frequency distributions. The results of this study show that the fishing gear can be size selective depending on hook size. The fitted selectivity models for each experiments were very different despite having two hooks in common. This is probably due to the very different catch size distributions in the two periods, which suggests that the population size structure changed significantly between 2000/2001 and 2004/2005.