ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on March 5, 2008
ICES Journal of Marine Science: Journal du Conseil 2008 65(4):560-570; doi:10.1093/icesjms/fsn015
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Biology of the skates Bathyraja brachyurops and B. griseocauda in waters around the Falkland Islands, Southwest Atlantic
Fisheries Department, Falkland Islands Government, PO Box 598, Stanley, Falkland Islands
Correspondence to A. I. Arkhipkin: tel: +500 27260; fax: +500 27265; e-mail: aarkhipkin{at}fisheries.gov.fk
Arkhipkin, A. I., Baumgartner, N., Brickle, P., Laptikhovsky, V. V., Pompert, J. H. W., and Shcherbich, Z. N. 2008. Biology of the skates Bathyraja brachyurops and B. griseocauda in waters around the Falkland Islands, Southwest Atlantic. – ICES Journal of Marine Science, 65: 560–570.The distributions and length compositions of two large, abundant skates, broadnose skate Bathyraja brachyurops and grey-tail skate B. griseocauda, were studied using an extensive dataset (
48 000 skates) collected from the commercial fishery and research cruises on the southeastern Patagonian Shelf around the Falkland Islands between 1992 and 2006. Bathyraja brachyurops mostly inhabit the shelf at depths shallower than 250 m, whereas B. griseocauda are found deeper (>250 m), off the shelf break and slope. Small individuals of both species are most segregated spatially. Growth increments on caudal thorns and vertebral centra revealed that B. brachyurops grow slower and have a shorter lifespan, attaining a maximum total length of 125 cm at 20 years, compared with B. griseocauda (150 cm at 28 years). Maturity is attained at 8–10 years for male and female B. brachyurops, and at 15 and 17 years for B. griseocauda, respectively. Spawning takes place on spatially segregated spawning grounds, B. brachyurops reproducing above the shelf break, and B. griseocauda just below the shelf break throughout the year, with a smaller proportion of females of both species laying eggs in winter.
Keywords: age, Bathyraja brachyurops, Bathyraja griseocauda, reproduction, skate
Received 3 July 2007; accepted 16 January 2008; advance access publication 5 March 2008.
 |
Introduction |
|---|
 Top Introduction Material and methodsResultsDiscussionReferences |
|---|
The Patagonian shelf and continental slope are among the few places in the world’s oceans with a great biodiversity of skates and rays. Some 20 species of rajid belonging to six genera inhabit the waters around the eastern part Falkland Islands (Agnew et al., 1999). Several large skates (Dipturus chilensis, Bathyraja brachyurops, B. griseocauda, and Bathyraja albomaculata) are abundant and have become the target of commercial fisheries. Initially (in 1986), they were taken as bycatch in the finfish trawl fishery. Since 1989, however, they have been caught by a specialized Korean trawl fleet, with total annual catches of between 4000 and 5000 t (Agnew et al., 2000). Commercial catches are of mixed species, approximately two-thirds consisting of B. albomaculata, B. brachyurops, and B. griseocauda. This large skate fishery requires effective management in order for it to be sustainable, and a multispecies approach to estimate their stock size was suggested by Agnew et al. (1999, 2000), with further revision by Wakeford et al. (2005).
Despite being exploited for some 20 years, very little is known about the biology of these skates. Most of the scientific literature where both B. griseocauda and B. brachyurops are mentioned are species lists and identification keys (McEachran and Dunn, 1998; Compagno, 1999). A study of the feeding spectra of these skates around the Falkland Islands revealed that both skates are opportunistic predators feeding on a variety of demersal prey (Brickle et al., 2003). The shallower water B. brachyurops is euryphagous and feeds on squid, fish, and crustaceans, whereas the deeper water B. griseocauda is predominantly piscivorous and the more active predator of the two.
Hard structures of both skates (caudal thorns and vertebral centra) can be used to determine the age of skates (Gallagher and Nolan, 1999). Marginal increment analysis shows that thorn growth decreases in winter, forming a distinct check on the thorn surface. This, combined with the good correspondence between counts of growth bands in vertebral centra and thorns, indicates that both structures can be used to age these skates, with the thorn technique being simpler and less time-consuming than a technique involving vertebrae.
The aims of the present study were to investigate the spatial and vertical distributions, and the spawning seasons, and to study age, growth, and maturation of B. brachyurops and B. griseocauda with a view to establishing possible ecological interactions between the two species off the Falkland Islands.
|
Material and methods |
|---|
|
TopIntroductionMaterial and methodsResultsDiscussionReferences |
|---|
Biological sampling
Two sampling schemes were used to collect specimens. First, skates were collected by scientific observers from the Falkland Islands Fisheries Department (FIFD) accompanying specialized skate trawlers between 1992 and 2006. For each trawl, the start position was recorded to at least the nearest minute of latitude/longitude, and the modal fishing depth over the course of the trawl was estimated. Observers sampled two trawls per day. The objective of this sampling scheme was to identify and measure the disc width, DW, of all skates from the catch. In all, 25 181 B. brachyurops and 20 469 B. griseocauda were sampled in this manner. As the observers were unable to dissect the skates on board because of the resulting loss of commercial value, the sex was only identified externally without any assignment of maturity stage. This sample set was only used to analyse spatial and bathymetric distributions.
The second sample set was collected by observers both from the skate bycatch during the bottom-trawl fishery for demersal fish and from research cruises on board the Falkland Fishery Patrol and Research Vessel "Dorada" between 1996 and 2006. Two trawls per day were sampled on board commercial vessels, and all trawls were sampled during the research cruises. For all skates, total length (LT) and disc width (DW) were measured to the nearest centimetre, and body weights (BW) to the nearest gramme. The skates were then cut open, and their sex and maturity stages assigned using the six-stage maturity scale developed by FIFD (Table 1). Totals of 1242 B. brachyurops and 981 B. griseocauda were sampled using this scheme, and these data were used to analyse age, growth, and reproductive biology.
|
Historically in the Falkland Islands fishery, DW has been used as the length measurement of skates during sampling on board the specialized skate trawlers. However, LT rather than DW is widely used in biological studies of skates elsewhere. To allow for our length frequency data on bathymetric and spatial distribution to be comparable with other studies, DW was transformed to LT using data derived from field measurements:
|
|
|
|
To estimate possible deviation in sex ratios from equality, only catches where >50 skates were sampled were taken into account (102 hauls for B. brachyurops, 149 hauls for B. griseocauda).
Age sample collection
Multiple methods are recommended for age determination of chondrichthyan fish (see the review by Cailliet et al., 2006), so it was decided to collect both thorns and vertebrae from the same animals for subsequent age estimation. Initially, thorns and vertebrae were collected opportunistically, but later in the study, the size groups with small sample numbers were targeted in an attempt to increase their sample sizes (up to 10 animals of each sex per 5 cm LT interval). In all, 260 B. brachyurops (15–82 cm LT) and 254 B. griseocauda (23–135 cm LT) were sampled. On-board ship, between 10 and 15 caudal thorns were cut from the tail posterior to the first haemal arch, and stored in a plastic sample bag. Also, two cuts were made on either side of the vertebral column from the scapulocaracoid in a posterior direction to remove between 5 and 10 vertebrae. The vertebrae were placed in the same plastic bag as the thorns, and the whole sample labelled with relevant station information and biometric data. The samples were then deep-frozen for further processing in the laboratory.
Thorn and vertebra processing and reading
Once in the laboratory, the thorns were soaked in hot water (95°C) for 10 min. This loosened the skin and flesh, making it easier to remove. Once the skin and flesh had been mechanically cleaned from the thorns, they were inspected for damage; damaged thorns were discarded. The remainder were put into a 5–10% trypsin solution at 37°C for 24–48 h, depending on their size, to remove any traces of flesh. Then, the thorns were rinsed with fresh water and air-dried for up to 3 d. The completely dried thorns were etched in 5% EDTA for 10 min, rinsed in fresh water, then microwaved on full power for 3 min, to enhance the etching and dehydration process. The thorns were then placed in a 1% silver nitrate solution for 45–60 min, rinsed in water, then dried with blotting paper and exposed to ultraviolet light until they were suitably blackened. The staining process was halted by immersing the thorns in a 5% sodium thiosulphate solution for 5 min, then leaving them to dry in air. The ridges on the thorns were read using a dissecting microscope with reflected light. The process follows that of Gallagher and Nolan (1999).
A similar process was used to clean the vertebrae. Initially, the vertebrae were soaked in hot water (100°C), then they were cleaned by hand to remove the flesh and excess cartilage. After this, they were treated in a trypsin solution similar to that of the thorns, then washed and air-dried for up to 1 week. The individual vertebral centra were then mounted on a microscope slide, and the slide placed on a hot plate with a few crystals of thermoplastic cement (Crystal BondTM, Aremco Products, Inc.) until the cement melted. The centrum was set in the melted cement, positioned sideways, and ground on one side almost until its core. It was then flipped over and ground from the other side, effectively producing a "bow tie" longitudinal section. Grinding was initially with 320-grit sandpaper, but the material was finally polished with 800-grit sandpaper. A drop of immersion oil was placed on the ground centrum, and the growth increments were read with both transmitted and reflected light under a dissecting microscope.
The clarity of the ridge-defined increments on the thorns varied greatly, but most thorns from both species showed clear ridges (Figure 1). The patterns of increments in the vertebrae of both species were less obvious, more difficult to discern (Figure 1), so a mixture of transmitted and reflected light had to be used to see them. As a result of this, reading the ridges on the thorns was considered to be the most convenient method for age estimation for both these species.
|
Validation was not attempted in this study, because the marginal increment analysis of both thorns and vertebrae had been conducted already for both species (Gallagher and Nolan, 1999). The location of the hatch mark (age zero) was defined from recently hatched juveniles of 15 and 19 cm LT for B. brachyurops, and of 21 and 24 cm LT for B. griseocauda. The hatch mark on vertebrae was considered to be the first mark after the change in angle of the corpus calcareum (Gallagher et al., 2006).
Three readers (R1, R2, and R3) read the whole sample set of thorns for both species three times. R1 was considered to be the most experienced, because this individual had previous experience in processing and reading thorns from related skate species in the Falkland Islands. Inter- and intra-reader data were compared using the average percent error (APE; Beamish and Fournier, 1981):
|
|
For R2 and R3, comparisons between their first and second readings showed poor precision, with APE values >10. Comparisons between their second and third reading resulted in much better precision (APEs between 5 and 10). Therefore, their first and second readings were considered to be part of a training set, and only the third readings were used subsequently for analysis. The third readings of each reader were compared for inter-reader precision.
Readings from thorns and their corresponding centra were also compared using APE, a paired t-test, and simple linear regression.
Samples were also selected to assess the variation of intra-sample increment numbers for thorns. Up to 20 thorns from each skate were counted and compared, and the results presented as charts of thorn number against age for each species.
Growth rate estimation
The von Bertalanffy growth model was fitted using a non-linear least squares regression using the third reading of R1:
|
|
Changes in LT were plotted against age interval to illustrate absolute growth rates. The size at birth (L0) was calculated as
|
|
Length and age at maturity
Skates were considered to be mature from stage III (Table 1). Stage III females had ovaries containing developing oocytes which were predominately translucent. The males at stage III were characterized by claspers that were not quite ridged. Internally, their testes were typically swollen, but their sperm ducts were not filled with sperm. It is considered that individual skate return to stage III after mating and egg laying.
To estimate the age and length at 50% maturity (AM50, LM50), a three-parameter logistic model was fitted:
|
|
|
Results |
|---|
|
TopIntroductionMaterial and methodsResultsDiscussionReferences |
|---|
Spatial and bathymetric distributions
Both species were found over the entire Falkland shelf (Figure 2). However, densities differed on the fishing grounds along the shelf edge, B. griseocauda tending to concentrate in the western and more deep-water part, and B. brachyurops being most abundant in the eastern shallow-water part.
|
Bathyraja brachyurops was taken between 33 and 503 m deep, but mostly between 100 and 300 m. Juveniles and subadults (10–30 cm DW, 15–40 cm LT) were caught on the outer shelf (100–200 m), smaller animals inhabiting shallower water. Larger skates (30–40 cm DW, 40–50 cm LT) were distributed over the entire depth range of the species, with the largest animals dominating at 300–350 m over the continental slope (Figure 3a). Bathyraja griseocauda was found between 106 and 1010 m deep. After hatching, juveniles of 10–20 cm DW (15–30 cm LT) move from their nursery grounds located at
300–350 m to deeper water (400–600 m). Upon reaching 20–30 cm DW (40–45 cm LT), they generally move to the upper slope (200–400 m), but some animals migrate to water deeper than 600 m. Large skates (80–90 cm DW, 105–130 cm LT) dominated the catches of the species between 400 and 600 m deep. The data suggest that hatchlings co-occur with skates of medium size, and early juveniles with large skates (Figure 3a).
|
The bathymetric segregation between species is strongest for juveniles. In all size groups, B. brachyurops was shallower than B. griseocauda. The proportions by weight of both species in commercial catches (Figure 3b) confirm that they are segregated by depth. Assuming similar catch rates for both species, it can be estimated that some 80% of the combined biomass of both species is represented by B. brachyurops shallower than 250 m, and by B. griseocauda deeper than 300 m.
Size and sex ratio
The total length of the largest female B. brachyurops sampled by the research vessel was smaller than in commercial catches (90 cm vs. 125 cm LT, respectively), whereas males were of similar length (89 and 90 cm LT, respectively). For B. griseocauda, the largest female sampled during a research cruise was 110 cm DW (157 cm LT) and in commercial catches 118 cm DW; for males, the values were 98 and 118 cm DW, respectively.
The bulk of the catch from the specialized skate fishery in the Falkland waters consisted of small skates (Figure 4). As the sampling rate was rather small and inconsistent during the first years of the fishery, length frequency distributions in catches are presented only for years with an annual sample size of >500 skates, corresponding to the period 1996–2006. Modal lengths of skates were variable, without any clear trend through the years (Figure 4). However, B. griseocauda usually had smaller modes (DW 25–40 cm) than B. brachyurops (DW 25–55 cm).
|
Except the largest size groups, in which females dominated, the sex ratio had a slight prevalence of females (Figure 5), 53.9% in B. brachyurops of 10–60 cm DW (t = 56.98, d.f. = 101, p < 0.0001), and 53.6% in B. griseocauda of 10–80 cm DW (t = 65.23, d.f. = 148, p < 0.0001).
|
Spawning seasonality and habitat
Mature female B. brachyurops with fully formed pre-spawning egg capsules were found throughout the year except in December, suggesting that spawning takes place year-round, with weak spring and autumn peaks (Figure 6a). They were found at depths of 33–310 m (mean 157 m), but most were between 200 and 300 m (Figure 6c). Hatchlings of 8–8.5 cm DW (12–14 cm LT) were caught in July, September, and October between 123 and 175 m deep. Hatchlings and post-hatchlings of 8–10 cm DW (12–16 cm LT) were distributed all round the Falkland Islands between 110 and 296 m deep (mean 153 m).
|
Mature female B. griseocauda with fully formed egg capsules were found at a greater depth than those of B. brachyurops, between 155 and 416 m deep (mean 263 m), with the bulk of them (81.5%) between 180 and 310 m. They were encountered in every month except January, with a weak spring/summer spawning peak (Figure 6b and d). Hatchlings and post-hatchlings of 10–12 cm DW (15–20 cm LT) were caught in May and between August and November, between 208 and 365 m deep (mean 268 m), mainly north of 50°S.
Taking into account the occurrence of hatchlings and females with fully developed egg capsules and hatchlings, we deduce that the spawning grounds of B. brachyurops are located on the shelf all round the Falkland Islands between 100 and 200 m deep. The spawning grounds of B. griseocauda are deeper and closer to the shelf edge between 200 and 300 m, leaving a wide gap to the northwest of the Falkland Islands. The mean post-hatchling and spawning female depths were statistically different between species (t = 9.262, p < 0.0001, and t = 7.569, p < 0.0001, respectively).
Precision of age determination, and a comparison of methods
The APE of subsequent readings decreased for reader R1 (Table 2). The readings of R1 and R3 for thorns showed the closest agreement for both species (within 3% and 2.4% for B. brachyurops and B. griseocauda, respectively); both readers were less in agreement with R2. This suggests that R2 had the lowest band-resolving ability. Consequently, only the results of the third readings of reader R1 were used as age estimates for both species of skates in this study.
|
To evaluate thorn variability on the tail of a single animal, all caudal thorns from 6 B. brachyurops and 21 B. griseocauda were read by R1 (Figure 7). Consistently and for both species, the first anterior thorns resulted in lower readings. This suggests that the anterior caudal thorns were more prone to damage than those in the posterior part of the tail, highlighting the importance of collecting thorns on the tail posterior to the haemal arch.
|
Comparisons between readings of thorns and vertebral increments for B. brachyurops resulted in a relatively high APE (15%). Closer examination suggested that reading vertebrae underestimated the age of this species (Figure 8a), a suspicion confirmed by a significant difference in the means (t = 9.68, d.f. = 255, p < 0.0001). As the APE for thorns from the two last readings of reader R1 were low, we conclude that the vertebrae of B. brachyurops are more difficult to interpret than the thorns. Comparisons of thorns and vertebrae in B. griseocauda, on the other hand, resulted in a lower APE (8.7%). The errors in interpretation were relatively evenly distributed above and below the line of equality (Figure 8b), and the t-test for differences in means was not significant (t = 1.09, d.f. = 257, p > 0.05).
|
Age, growth, and maturity
Bathyraja brachyurops are relatively slow-growing (Table 3). Combining the data for males and females yielded mean absolute growth rates of 5.1, 3.3, and 2.4 cm LT year–1 for the first 9, 9–14, and 14–20 years, respectively. Females initially (up to
10 years) grew faster than males, but thereafter male growth rates overtook those of females. Females also attained a larger LT than males (Figure 9a and b). The maximum age observed for the species was 20 years.
|
|
Absolute B. griseocauda growth rates were faster than for B. brachyurops, so the species attained a greater maximum size (Table 3). The growth rates for B. griseocauda decreased from 5.6 cm LT year–1 for the first 9 years to 4.3 cm LT year–1 between 14 and 20 years (Figure 9c and d). The maximum age observed for this species was 28 years.
Length/weight relationships of female and male B. brachyurops and B. griseocauda are provided in Table 3.
Calculated values of L0 (length at birth) and those from field observations were very close to the size at which the animals apparently hatch: 15 and 20 cm LT for B. brachyurops and B. griseocauda, respectively (Table 3).
The LM50 values for male and female B. brachyurops were 63.9 and 57.9 cm LT, respectively. Ages at maturity were 10.0 and 8.2 years for males and females, illustrating the late maturation of the species (Figure 10). Bathyraja griseocauda matured at a larger size and a later age, with LM50 values of 108.2 and 94.5 cm LT and ages at maturity of 17.8 and 14 years for females and males, respectively (Figure 10).
|
|
Discussion |
|---|
|
TopIntroductionMaterial and methodsResultsDiscussionReferences |
|---|
Spatial distributions suggested that both B. griseocauda and B. brachyurops are common inhabitants of the shelf and continental slope around the Falkland Islands, B. brachyurops mostly inhabiting the shelf, and B. griseocauda mostly over the slope. Taking into account the mean depths of occurrence of both gravid females and small juveniles, the spawning grounds of both species are also spatially segregated, B. brachyurops reproducing above the shelf break, and B. griseocauda reproducing deeper, just below the shelf break. Because of this, it is possible that the latter species does not spawn between the western and eastern parts of the Patagonian shelf, where the depth is <150 m. Bathymetric segregation is strongest between juvenile skates of both species, and might have evolved to avoid competition for food, because diet overlap is great (84.5%), decreasing to 22–28% in skates >30 cm DW (Brickle et al., 2003).
Similar to other oviparous sharks and skates (Hamlett and Koob, 1999), both species are characterized by year-round spawning, with females laying fewer eggs in winter. Both thorns and vertebrae were initially assessed for age estimation of B. brachyurops and B. griseocauda. Following Gallagher and Nolan (1999), growth increments on thorns were found to be the most reliable for age estimation, even though their clarity varied among individual skate. Compared with thorns, the growth increments in vertebral centra were more difficult to discern, especially in B. brachyurops, so causing an underestimation of age in older animals. One of the reasons for such poor definition of growth bands in vertebrae could be the reduced calcium deposition in older animals, probably as a consequence of their diet, as recorded for D. chilensis by Licandeo et al. (2006). Ridges on thorns and bands in the vertebrae of both species were considered to be laid down annually, after marginal increment analysis carried out recently by Gallagher and Nolan (1999). An individual of the closely related B. cousseauae, which has overlapping habitat, was marked by oxytetracycline (OTC), tagged, and subsequently recovered 10 months later. The presence of the fluorescent mark on its thorns and vertebrae suggested that the narrow ridge and band, respectively, were formed in winter (Gallagher and Nolan, 1999). Hopefully, a tag-recapture programme conducted by FIFD on the Falkland Islands Shelf will result in further OTC validation of both species of skate investigated here.
The readings of R1, based on the examination of thorns in the present study, suggest that precision increased with practice. Among three readers, the closest agreement was observed between R1 and R3 (
3% error). Readings between the other readers and R2 showed poor agreement (>9% error), indicating that R2 had poor resolving abilities for growth ridges, which highlights the need for appropriate training before reading. Readings taken from thorns of the same skates revealed that thorns anterior to the haemal arch had fewer growth ridges and were more prone to damage than the posterior thorns in the caudal region. When wanting to use thorns for age estimation of skates, we suggest that all thorns from one fish should be examined initially to ascertain the location of the most suitable thorns. For B. brachyurops and B. griseocauda, undamaged thorns from the anterior part of the tail yielded the highest counts.
The life histories of elasmobranchs are generally characterized by an equilibrium strategy (large-sized fish with late maturation, low fecundity, and a high degree of parental investment; King and McFarlane, 2003), and the skates studied are no exception to this rule. Both B. brachyurops and B. griseocauda are relatively slow-growing and long-lived (with maximum ages of >20 years), the latter species having higher absolute growth rates. Their age and rates of growth are comparable with skates living in similar environments. For example, Bathyraja aleutica from Alaskan waters attains 150 cm LT at a maximum age of 19 years. Raja binoculata and Raja rhina from the same cold-water habitat live for 13 and 15 years, respectively, and attain 160 cm LT, therefore having even faster growth rates than the large Falkland skates (Gburski and Foy, 2004). The large skates Dipturus trachyderma (26 years) and Dipturus chilensis (21 years) from the Southeast Pacific (Licandeo et al., 2006, 2007), which also occur in waters around the Falkland Islands, have quite similar maximum age to that of both species studied here.
As in most skates (Francis and Ó Maolagáin, 2005; Licandeo et al., 2007), growth in length was well described by the von Bertalanffy growth function (VBGF). Estimated parameters of the VBGF for B. brachyurops seem to be more realistic than for B. griseocauda, because the values of L
for both sexes in the former species are quite close to the maximum sizes observed in catches (125 cm LT). In contrast, the L for B. griseocauda was much larger than the maximum observed LT (200 cm), attributable perhaps to poor resolution of growth increments on the thorns of large fish (causing underestimation of age) or (more likely) the skewed age/length composition of samples. Most of the B. griseocauda were small or of medium size, and few were sufficiently large to have artificially inflated the L, highlighting the importance of collecting and reading sufficient numbers of thorns at either end of the size spectrum. As suggested by Cailliet et al. (2006), the values for L0 were calculated from the parameters of the VBGF and were similar to the sizes of newly hatched skates encountered in the field, B. brachyurops and B. griseocauda hatching at 15 and 21 cm LT, respectively.
Our results showed that both skate species studied here are late-maturing. Indeed, compared with other large skates inhabiting the southern Atlantic and southern Pacific, e.g. D. chilensis and D. trachyderma, both could be considered among the most late-maturing skates (Licandeo et al., 2006, 2007) in the family. Some cold-water skates mature earlier than these Falkland skates, e.g. Amblyraja georgiana from the Ross Sea attains 14 years (at a pelvic length of 70 cm) with males attaining maturity at 6–7 years and females 8–11 years (Francis and Ó Maolagáin, 2005).
With the recent development of high-tech trawl and longline fisheries, marine top predators such as elasmobranchs are overfished by many fisheries (Pauly et al., 1998). In contrast to this situation, the Falkland Islands’ skate fishery appears resilient to current fisheries management (Agnew et al., 1999, 2000). More than 15 years of continuous exploitation of the stocks of slow-growing, late-maturing, low-fecund skates has not seemed to impact the populations, and catches have remained reasonably stable at 4000–5000 t per annum (all species of skate), without any obvious trends in rates of maturity or decreasing size. It seems that several factors may contribute to the successful exploitation and management of the skate resources around the Falkland Islands. First, the fishing grounds are relatively compact and restricted to depths of 200–300 m to the north of the archipelago, suggesting that the spawning grounds of both skates are not being seriously impacted by the fishery. Second, the fishery exploits the feeding grounds populated mainly by either a significant proportion (B. brachyurops) or almost exclusively (B. griseocauda) immature skates, leaving adult and mature parts of the populations to spawn naturally. Third, usage of specialized bottom trawls with a tickler chain is prohibited elsewhere on the Falkland Shelf, allowing the latter to function like a marine protected area (MPA) for skates. Despite recent critiques of MPAs as tools for protecting fish stocks (Longhurst, 2006), the system seems to be working effectively for skate populations in the Falkland Islands, its effectiveness probably depending on the proportions of the population inhabiting the protected area and migrating from there to the fishing grounds.
We hope that the analysis contained in this manuscript will contribute to the knowledge of basic biological parameters of these two abundant skate species around the Falkland Islands, as well as underpin even more effective management of the fishery and the stocks.
|
Acknowledgements |
|---|
We gratefully acknowledge the work of the scientific observers of the Falkland Islands Government, Fisheries Department, who collected samples from the skate fishery. We also thank the Director of Fisheries, John Barton, for supporting the work. A part of the analysis was presented at the Annual Scientific Conference of the International Council for Exploration of Sea (ICES) held in September 2005 in Aberdeen, UK, as ICES Documents CM 2005/N: 2 and CM 2005/N: 14.
|
References |
|---|
|
TopIntroductionMaterial and methodsResultsDiscussionReferences |
|---|
-
Agnew D. J., Nolan C. P., Beddington J. R., Baranowski R. Approaches to the assessment and management of the multispecies skate and ray fishery using the Falkland Islands fishery as an example. Canadian Journal of Fisheries and Aquatic Sciences (2000) 57:429–440.
Agnew D. J., Nolan C. P., Pompert J. Management of the Falkland Islands skate and ray fishery. In: Case Studies of the Management of Elasmobranch Fisheries—Shotton R., ed. (1999) Rome: FAO. 268–284. 904 pp.
Beamish J. R., Fournier D. A. A method of comparing the precision of a set of age determinations. Canadian Journal of Fisheries and Aquatic Sciences (1981) 38:982–983.
Brickle P., Laptikhovsky V., Pompert J., Bishop A. Ontogenetic changes in the feeding habits and dietary overlap between three abundant rajid species on the Falkland Islands’ shelf. Journal of the Marine Biological Association of the UK (2003) 86:1119–1125.
Cailliet G. M., Smith W. D., Mollet H. F., Goldman K. J. Age and growth studies of chondrichthyan fishes: the need for consistency in terminology, verification, validation and growth function fitting. Environmental Biology of Fishes (2006) 77:211–228.[CrossRef][Web of Science]
Compagno L. J. V. Checklist of living elasmobranchs. In: Sharks, Skates and Rays: the Biology of Elasmobranch Fishes—Hamlett W. C., ed. (1999) MD: John Hopkins University Press. 471–498. 516 pp.
Francis M. P., Ó Maolagáin C. Age and growth of the Antarctic skate (Amblyraja georgiana) in the Ross Sea. CCAMLR Science (2005) 12:183–194.[Web of Science]
Gallagher M., Nolan C. P. A novel method for the estimation of age and growth in rajids using caudal thorns. Canadian Journal of Fisheries and Aquatic Sciences (1999) 56:1590–1599.
Gallagher M. J., Green M. J., Nolan C. P. The potential use of caudal thorns as a non-invasive ageing structure in the thorny skate (Amblyraja radiate Donovan, 1808). Environmental Biology of Fishes (2006) 77:265–272.[CrossRef][Web of Science]
Gburski C., Foy D. Preliminary age and growth of skates (Raja and Bathyraja species) in the southern Alaska peninsula using vertebrae: big skate (Raja binoculata), longnose skate (Raja rhina), Aleutian skate (Bathyraja aleutica) and Bering skate (Bathyraja interrupta). (2004) Electronic reference. fttp://ftp.afse.noaa.gov/poters/pGburski01_skates.pdf.
Hamlett W. C., Koob T. J. Female reproductive system. In: Sharks, Skates and Rays: the Biology of Elasmobranch Fishes—Hamlett W. C., ed. (1999) MD: John Hopkins University Press. 398–443. 516 pp.
King J. R., McFarlane G. A. Marine fish life history strategies: applications to fishery management. Fisheries Management and Ecology (2003) 10:249–264.[CrossRef][Web of Science]
Licandeo R., Cerna F., Céspedes R. Age, growth and reproduction of the roughskin skate, Dipturus trachyderma, from the southeastern Pacific. ICES Journal of Marine Science (2007) 64:141–148.
Licandeo R. R., Lamilla J. G., Rubilar P. G., Vega R. M. Age, growth and sexual maturity of the yellownose skate Dipturus chilensis in the south-eastern Pacific. Journal of Fish Biology (2006) 68:488–506.[CrossRef][Web of Science]
Longhurst A. The sustainability myth. Fisheries Research (2006) 81:107–112.[CrossRef][Web of Science]
McEachran J. D., Dunn K. A. Phylogenetic analysis of skates, a morphologically conservative clade of elasmobranchs (Chondrichthyes: Rajidae). Copeia (1998) 2:271–290.
Pauly D., Christensen V., Dalsgaard J., Froese R., Torres F. Fishing down marine food webs. Science (1998) 279:860–863.
Wakeford R. C., Agnew D. J., Middleton D. A. J., Pompert J. H. W., Laptikhovsky V. V. Management of the Falkland Islands multispecies ray fishery: is species-specific management required? Journal of Northwest Atlantic Fisheries Science (2005) 35:309–324.
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||