Bycatch of lined seahorses in a Gulf of Mexico shrimp trawl fishery – Hippocompus erectus
Julia K. Baum
Abstract–Bycatch studies have largely ignored population level effects on fish species of little commercial interest. Here we analyze bycatch of the lined seahorse (Hippocampus erectus) in the bait-shrimp trawl fishery in Hernando Beach, Florida, providing the first fisheries data for this species. Based on catch per unit of effort (CPUE), size, sex, and reproductive status of trawled H. erectus, 1) approximately 72,000 seahorses were caught annually by this fleet, from a population of unknown size, 2) trawling affected population cohorts differentially because of temporal and spatial variation in CPUE and population size, and 3) a greater proportion of females than males was removed in trawling. Our findings suggest that trawling may affect seahorse populations through direct mortality, social disruption, and habitat damage. However, the lack of specific abundance or catchability estimates for H. erectus means that the precise impact of trawling on this fish remains uncertain. This paper focuses attention on the need for research and monitoring of small fishes that are caught incidentally in nonselective gear.
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The incidental capture of marine organisms is now recognized as a serious problem in fisheries management and marine conservation (Alverson et al., 1994; IUCN, 1996; Alverson, 1997; Jennings and Kaiser, 1998; FAO, 1999). Shrimp trawl fisheries are the single greatest source of bycatch, accounting for 35% of the world’s total bycatch (Alverson et al., 1994). Bycatch research has focused on marine megafauna, seabirds, and commercially important fish species (see as examples Polacheck, 1989; Graham, 1995; Weimerskirch et al., 1997; Julian and Beeson, 1998; Pikitch et al., 1998; Galloway and Cole, 1999; Diamond et al., 2000). The conservation impacts of bycatch for noncommercial fishes and invertebrate species remain largely unstudied (but see Chan and Liew, 1986; Pettovello, 1999; Milton, 2001). The few studies that have evaluated incidental capture of these species have focused on survival rates of individuals (Hill and Wassenberg, 1990, 2000; Kaiser and Spencer, 1995; Probert et al., 1997; Mensink et al., 2000) without addressing population level effects of bycatch. However, even species that comprise only a small portion of the bycatch in a fishery may experience significant impacts of incidental harvest on their population size and structure.
Seahorses are among those species inferred to be greatly affected by nonselective fishing gear, both because intense trawling often covers seahorse habitat and because their life history traits likely render these fishes vulnerable to overexploitation (Vincent, 1996). Most studied seahorse species are strictly monogamous (i.e. sexually and socially), meaning that removal can disrupt pairs and may reduce reproductive output (e.g. Vincent, 1995; Vincent and Sadler, 1995; Kvarnemo et al., 2000; Perante et al., 2002). Obligatory parental care by males, combined with relatively low fecundity, may reduce the potential for population recovery from overexploitation, although potentially high survival of young may also offset this apparent cost. In addition, sparse distributions and low mobility suggest that seahorses will be slow to recolonize depleted areas (Perante et al., 2002; Vincent et al. (1)).
Seahorses derived from bycatch appear to be contributing greatly to the large and growing international trade in these fishes (Vincent, 1996; Vincent and Perry (2). Consumer demand for seahorses–both dried for traditional medicine and curiosity trades, and, less frequently, live for the aquarium trade–is very high (Vincent, 1996). Global demand for seahorses has surpassed supply and therefore trade has increased and expanded geographically (Vincent, 1996; Vincent and Perry (2)), placing populations around the world under greater pressure. Much of this market demand is met from retention of incidental landings in shrimp trawls. Where no market has yet developed, incidentally caught seahorses are discarded and the survival rate for these discarded seahorses is unknown.
Trade records and anecdotal evidence from other countries indicate that the United States has imported and exported considerable numbers of both live and dried seahorses in recent years (Vincent, 1996; Vincent and Perry (2)). In Florida, the primary source of live seahorses in the United States (Larkin and Degner, 2001), these fishes ranked as the seventh most economically important ornamental fish group from 1990 to 1998, and seahorse landings rose 184% during this period, whereas landings of each of the more valuable fish groups declined (Adams et al., 2001). Many seahorses in the United States are probably obtained from bycatch of the shrimp trawl fisheries that operate in known seahorse habitats along the Atlantic coast of the United States.
Our objective was to document bycatch of the lined seahorse (Hippocampus erectus) in a live-bait shrimp trawl fishery. This fish is often retained for the aquarium trade. We quantify number, sex, size, and reproductive status of trawl-caught seahorses and examine how these parameters vary temporally and spatially. We also comment on potential conservation concerns resulting from this fishery.
Materials and methods
We focused our study on the lined seahorse (Hippocampus erectus) because it was caught much more often than its sympatric congenerics in the Gulf of Mexico–the longsnout seahorse (H. reidi) and the dwarf seahorse (H. zosterae). Hippocampus erectus is a large, deep-bodied seahorse (adult height 5.5 to 18.5 cm: Lourie et al., 1999) and has a geographic range that extends from southern Canada to Argentina. Most species of seahorse have short lifespans and low fecundity: Hippocampus erectus lives for about four years (Lourie et al., 1999) and has broods of about 100-1500 young (Teixeira and Musick, 2000). Hippocampus erectus is found in shallow waters and offshore to depths of over 70 m, primarily in mangroves and seagrass beds (Vari, 1982). Like many other seahorse species, H. erectus is listed as Vulnerable (A2cd) by the International Union for Conservation of Nature and Natural Resources (IUCN, 2002), based on suspected declines resulting from habitat degradation and exploitation. However, as is the case for many small fishes, there is little information known about the biology of H. erectus, and no fishery data exist for it.
Seahorse bycatch was assessed in the live-bait shrimp trawl fishery operating from Hernando Beach, Florida (Fig. 1). The fishery, using roller beam trawls, targets pink shrimp (Penaeus duorarum) at night in seagrass beds and relocates seasonally in Florida. Hernando Beach was chosen as our study site because it is a moderate-size fishing port with 31 licensed trawlers and is active during the summer sampling period. Boats were equipped with trawls that had a slotted roller along the bottom of the frame, and stainless steel finger bars attached vertically, 5 cm apart along the length of the frame, to limit collection of benthic substratum and other debris (Berkeley et al., 1985). Trawls were towed from each side of the vessel in four configurations: 1) one trawl per side, each measuring either 3.66 m, 4.27 m, or 4.88 m in length, or 2) two trawls per side, each measuring 3.66 m in length. Net mesh sizes were 3.18-3.81 cm and the tail bag stretched mesh size was 2.54-3.18 cm. Trawls usually lasted between 30 and 60 minutes, and fishermen made multiple successive trawls in a night. Shrimp were culled from the catch and held live onboard in aerated holding tanks. Most bycatch was discarded overboard, although some fishermen retained certain species, including seahorses, for sale as aquarium fishes.
Bycatch sampling and data set description
Data on seahorse bycatch in the live-bait shrimp fishery were collected on 95 fishing nights, from June to August 1998 and from June to July 1999, using three methods: 1) we sampled bycatch onboard on 50 nights; 2) we recorded seahorse bycatch data onshore on 14 nights from fishers who retained seahorses to sell; and 3) we received data from fishermen on their seahorse catches (including location, time, number of trawls, and number of seahorses per trawl) on a total of 31 nights. Onboard sampling was semistratified in that we targeted our sampling to cover all lunar and tidal phases and a variety of areas. However, we were dependent on fishersmen’s decisions about the time and location of tows. We also collected anecdotal information from 14 experienced fishermen about seahorse catches over time.
During onboard sampling, we recorded the number of seahorses caught, start and end time of trawl, depth and location (Loran co-ordinates) of trawl, tidal and lunar phases, and presence or absence of all bycatch species, including biogenic habitat species. Hippocampus erectus found in the catch were placed in a container of surface water while measurements were made. They were then released, except when fishermen chose to retain them for sale. We measured, weighed, determined the sex, and recorded the life history stage, reproductive status, and any injury for each seahorse. Measurements were taken according to Lourie et al. (1999) and included standard length, defined as the length from the tip of the snout to the opercular ridge and from the opercular ridge to straightened tail tip. Seahorses that had lost tail rings were not included in the length analysis. Unless precluded by logistic constraints, wet weights were obtained onboard by using a 60-g Pesola spring scale and onshore by using a 200-g Ohaus electronic balance. Adult males were distinguished from females and juveniles by the presence of a brood pouch. The standard length of the smallest seahorse with a brood pouch (105.3 ram) was used as the division between adults and juveniles with the assumption that males and females matured at the same size. Such an assumption may overestimate the number of adult females because males that matured after that size would have been included in our analysis as females. We defined males as reproductively active if they were pregnant or had recently released young (as indicated by a loose pouch). Female reproductive state was not included in this analysis because it is difficult to determine reliably. We defined mortality to include seahorses already dead when the net was hauled and those that died onboard.
We evaluated temporal and spatial patterns in catch per unit of effort (CPUE), standard length, population structure, and reproductive status of the seahorse bycatch. Specifically, we tested 1) temporal effects of year, lunar and tidal phase, and 2) spatial effects of area and depth. In our estimates of seahorse CPUE, we used standard length instead of biomass because female weight changes with egg hydration and male weight increases greatly when carrying embryos. Calculating CPUE per length (m) of roller beam trawl controlled for variation in gear size. The sampling unit was thus defined as the total or cumulative standard length of seahorses caught during each tow (per hour), per meter of trawl (per meter). Lunar phases were defined as continuous variables by converting lunar day to its angle, [theta], based on a cycle of 29.5 days (=360[degrees]), with new moon defined as [theta] = 0[degrees]. These angles were then converted to their cosine and sine functions for inclusion in linear regression (deBruyn and Meeuwig, 2001). Tides were semidiurnal in the Gulf of Mexico, ranging 1.3 m in tidal level in the study area. High and low tides were defined as each lasting two hours, with the remaining time classified as ebb or flood accordingly. Spatial effects were analyzed by dividing the total fishing ground into eight subareas according to their position with respect to the depth contour and by identifying discrete geographical clusters of trawls (Fig. 1). We then compared variation among fishing areas within years for those areas with at least five observations (areas I1, I2, N1, N2, S1, S2 in 1998, and areas N1, N3 and N4 in 1999). Interannual spatial comparisons were not possible because there was little overlap in sampled areas between the two years.
Statistical analysis
We based CPUE estimates on all trawls (n=445). Statistical analyses evaluating temporal and spatial variation in seahorse bycatch included only those trawls with nonzero seahorse observations (n=205). The analysis required the data or their residuals to be normal; this was achieved by inverse hyperbolic sine transformations following exclusion of zeros (Zar, 1996). It should be noted that by excluding zeros we overestimated seahorse CPUE and we lost information about areas where seahorses were absent or rare. Our analyses should be interpreted as applying to locations where and times when seahorses were found in sufficient numbers as to be caught.
We examined the data using ANOVA, ANCOVA, linear regression and chi-square analyses (Zar, 1996; SYSTAT, version 7.0, SPSS Inc., Chicago, IL). All two-way and three-way ANOVAs included tests for interactions. We used a general linear model because the data were unbalanced. Interactions were removed from the model if they were found to be nonsignificant. Models were then rerun, followed by pairwise Tukey tests to indicate where significant differences occurred (Zar, 1996; SYSTAT, version 7). We report results for the final ANOVA only. The Yates correction was applied to 2 x 2 chi-squares (Zar, 1996). All significance levels were set to reject [H.sub.0] at P<0.05 and all means are reported with standard errors.
Results
Bait shrimp fishermen trawled from 11 to 24 km offshore, between 1.8 and 6.4 m deep water (mean=3.76 [+ or -] 0.87 m). Fishermen typically left port between 17:00 and 19:00 and spent 5.8 [+ or -] 0.23 h actively trawling per night (n=50 nights). Trawls lasted 40.2 [+ or -] 11.4 min (n=445) on average, and fishermen usually set 8 to 9 trawls per night. Distance trawled could not be estimated because we were unable to track trawler trajectories continuously and because they changed direction during the tows. The benthic habitat was composed primarily of seagrass (Thalassia testudinum) but also included algae, coral, and sponge. Bycatch included at least 118 species of fishes, invertebrates, and marine flora.
Catch per unit of effort
Hippocampus erectus was the only seahorse species commonly caught in this fishery. Almost half of the trawls (46%) caught H. erectus, and the number per trawl ranged from 0 to 16,whereas no H. reidi and only two of the much smaller species, H. zosterae, were caught. In total, 916 H. erectus were caught during the 95 documented fishing boat nights of the two fishing seasons, resulting in an overall mean of 9.64 seahorses per fishing boat night. Mean CPUE for H. erectus was 24.25 [+ or -] 2.15 mm/(hxm), about one and a half seahorses per hour per boat. If only trawls with seahorses were included, CPUE was 52.52 [+ or -] 3.80 mnd(hxm), or about three seahorses per hour per boat (n=205). Very high CPUE was recorded on three nights: 16 July 1998 (mean CPUE=122.0 [+ or -] 22.5 mm/(hxm), n=12 trawls), 28 June 1999 (mean CPUE=118.1 [+ or -] 24. mm/(hxm), n=12 trawls) and 30 June 1999 (mean CPUE=154.9 [+ or -] 36.6 mm/(hxm), n=8 trawls). Bycatch is characterized by a high number of low catches and infrequent large catches. Because the large catches more likely reflect the spatial-temporal distribution characteristics of fish stocks rather than outliers of the data (Ortiz et al. 2000), we analyzed the entire dataset and then tested the robustness of our models by excluding these three nights in order to assess their influence on the CPUE patterns.
CPUE of nonzero trawls varied between years and with lunar phase (Table 1), but not with tidal phase (P=0.15). Trawls captured significantly more seahorses in 1999 than in 1998 (P<0.0005). The effect of lunar phase varied between years: CPUE was highest on the lunar third quarter in 1998, but only weakly significant and had slightly higher CPUE on the full moon in 1999 (Fig. 2). The temporal variation in CPUE was largely driven by the three high CPUE nights, but the effect persisted when these were excluded (Table 1).
[FIGURE 2 OMITTED]
In both years, there were significant differences in nonzero CPUE trawls among sites. In 1998, CPUE was significantly higher in S2 than in I2, N1, N2 and S1 (Fig. 3A, Table 1). In 1999, CPUE was significantly higher in area N1 than in areas N3 and N4 (Fig. 3B, Table 1). However, both of these spatial patterns, like the lunar patterns, were driven primarily by the high CPUE nights. Removing the three outliers left a significant difference in CPUE by area in 1998 only (Table 1). CPUE did not vary with depth of the fishing ground (P=0.67).
[FIGURE 3 OMITTED]
Size of seahorses
Mean standard length of adults (standard length [greater than or equal to] 105.3 mm) was 139.5 [+ or -] 21.7 mm (n=465, range 105.3-202 mm) and mean weight was 11.6 [+ or -] 5.5 g (n=232, range 3-31 g). Hippocampus erectus was sexually dimorphic: males had a brood pouch and were significantly longer than females (n=465, P<0.0005, Table 2), and had a greater weight to standard length ratio, although this latter difference was relatively weak (P=0.04). Juveniles (standard length <105.3 mm) had a mean standard length of 83.3 [+ or -] 16.7 mm (n=65, range 41.4-105 mm) and a mean weight of 2.4 [+ or -] 1.0 g (n=38, range 0.9-4.0 g).
In adults, standard length varied by year (n=425, F=7.1, P=0.008) and by lunar phase (n=425, cos([theta]):F=5.4, P=0.02, sin([theta]):F=7.7, P=0.006). Mean standard length was greater in 1998 than in 1999 and highest on the new moon. In 1998 significantly larger adult seahorses were caught in areas I2 and S2 than in I1 and S1 (n=229, F=13.7, P<0.0005; Fig. 4). There was no effect of area in 1999 (n=212, F=1.9, P=0.14). Standard length was not related to depth or tide.
Population structure
We assessed population structure in terms of size cohorts, life history stages, sex ratio (males/total) and reproductive status. Standard-length frequency histograms suggested that the 1998 bycatch was composed of three year classes but the largest of these was not evident in 1999 (Fig. 5). Of 530 seahorses measured, 87.7% were considered to be adults. The ratio of juveniles to adults did not differ by year (P>0.10) or tidal phase (P>0.90). However, significantly more juveniles were caught during the first quarter than during other lunar phases (Table 3). Fishing area had a significant effect on size class, with the highest numbers of juveniles caught in N2 and N3 (Table 3).
The sex ratio (males as fraction of total) of 0.42 differed significantly from a 1:1 ratio ([X.sub.c.sup.2]=19.56, df=1, P<0.001). The sex ratio did not vary temporally or spatially (Table 3), but it did vary as a function of size class. There were proportionally more males in the larger size class ([greater than or equal to]150 mm, 0.61) than in the smaller size class (<150 mm, 0.30) ([[chi square].sup.2]=14.95, df=1, P<0.005).
About 25% of the male seahorses captured in 1998 were considered to be reproductively active, whereas fewer than 1% were reproductively active in 1999. Indeed, male reproductive activity was higher in 1998 than 1999 even after controlling for smaller male size in 1999 (Table 3). The proportion of reproductively active males did not vary with lunar or tidal phase but did vary significantly with area (Table 3); almost half of the reproductively active males were found in one area (S2), and 83% of them were caught on one of the three nights with very high CPUE (16 July 1998).
Mortality and injury
Fewer than 1% of seahorses died during tows or sorting, but 4.7% (n=28 of 588 seahorses) lost tail rings. The mean loss was 22 of the usual 36 tail rings (Lourie et al., 1999), or 61% of the tail (range=8-31 rings lost). Approximately 14% of the losses (n=4 of 588 seahorses) appeared to be the result of new wounds, probably caused by the focal trawl. There was no effect of year (P=0.25), sex (P=0.75), or reproductive status (P=0.75) on incidence of seahorse injury. Postrelease mortality is unknown.
Discussion
Direct effects of the trawl fishery on seahorse mortality
We estimate that this fleet catches almost 72,000 seahorses incidentally per annum, based on the mean CPUE and given that 31 boats fished approximately 240 nights per year. Most seahorses were returned to the wild in the Hernando Beach fishery, but this may not be the case in other live-bait shrimp trawl fleets in Florida (Vincent, pers. obs.). We could not determine the potential fishing-induced mortality for the Hernandez Beach H. erectus population, even when all trawled seahorses were retained, because seahorse catchability in roller beam trawl gear is unknown and no studies have estimated H. erectus density in the wild. Focal behavioral studies on congenerics similar in size to H. erectus have found varied densities: H. comes, an exploited species associated with coral reefs in the tropics, had localized densities of 0.019/[m.sup.2] in a marine protected area, and much less elsewhere (Perante et al., 2002), and an unexploited species, H. whitei, had localized densities of 0.088-0.215/[m.sup.2] in a study area, and no seahorses were found over large adjacent areas (Vincent et al. (1)). Our study also suggests very patchy distributions of H. erectus (54% of the trawls had no seahorses at all and the number of seahorses per trawl set ranged from 0 to 16).
Although variation in CPUE may reflect differential catchability by habitat, we suggest that in areas where seahorses were caught, temporal rather than spatial effects drove CPUE. It is difficult to make conclusions about variation in CPUE because data were unbalanced, in that the areas trawled differed between years and among lunar phases. However, analysis of variance on a subset of data for three sites (N1, N2, N3) on three lunar phases (1st quarter, full moon, 3rd quarter) for which we had data in both years (n=149 trawls), indicated that there was a strong effect of year, a weaker effect of lunar phase, and no effect of site. These results suggest that CPUE was mainly affected by temporal variation. Lunar patterns in CPUE as a result of fish behavior and ecology are common (e.g. Parrish, 1999). This would be consistent with observations for other species of seahorses: H. comes in the Philippines (Vincent et al. (3)) and H. spinosissimus and H. trimaculatus in Vietnam (Meeuwig et al. (4)) exhibited patterns in CPUE with respect to lunar phase, although these species were also distributed in patches in space.
Data from this study suggest that the H. erectus population was spatially structured. In 1999, the mean size of incidentally caught adult seahorses decreased, reflecting the absence of the largest size class of males and an increase in smaller females that year (Fig. 5). We attribute this difference to spatial structuring: the shallower areas (I2, S2) where the largest male and female seahorses were caught in 1998 were not fished during the 1999 sampling season. Most of the seahorse bycatch were adult H. erectus; the dearth of juvenile H. erectus (and dwarf seahorses, H. zosterae) in the trawls reflects low catchability or retention due to mesh size. Similar proportions of juvenile seahorses were caught over the two sampling seasons. The ratio of juveniles to adults appears to be temporally influenced (proportionally more juveniles were caught on new moons), but this variation probably also reflects spatial structuring because these trawls occurred primarily in deeper offshore areas (N2, N3) that were fished almost exclusively during this lunar phase. Perhaps H. erectus undergoes ontogenetic movement, between juvenile and adult life history stages, and adults maintain site fidelity. Spatial size structuring probably also occurs in other seahorse species, for the entire population and for adults alone (H. comes, Meeuwig (5); H. guttulatus, Curtis (6)). A better understanding of the spatial structuring of populations could allow for spatial control of fishing effort to minimize bycatch.
We found a consistent, female-biased sex ratio in the catch across the two years of our study, with only 42% males. This bias may reflect the sex ratio of the H. erectus population: a similar sex ratio (40% males) was found in a population of H. erectus in Chesapeake Bay, Virginia (Teixeira and Musick, 2000). Female-biased sex ratios have also been found in H. zosterae (33% males) when sampled by pushnet (Strawn, 1958), and in H. abdominalis studied underwater in Australia (Martin-Smith (7). Many other wild populations of seahorses studied underwater, however, have documented equal numbers of males and females (H. breviceps: H. comes: Moreau and Vincent (8); Perante et al., 1998; H. reidi: Dauwe, 1993; H. whitei: Vincent and Sadler, 1995). Sexual dimorphism in H. erectus was too slight to explain different catchability of the two sexes and would, in any case, have favored the capture of males. The disproportionate catch of females could have arisen from spatial segregation by sex; the greater catches of reproductively active males in shallower areas suggests that males may spend most of their time inshore of the trawled area. We also cannot discount the possibility that some seahorses classified as females may have been immature males, and the sex ratio in the population could in fact be 1:1. The proportion of reproductively active seahorses in the bycatch was lower than expected, particularly in 1999. Our study occurred during summer, within the breeding season for the congeneric and sympatric H. zosterae in Florida (February to October; Strawn, 1958), and for H. erectus in Chesapeake Bay (May to October; Teixeira and Musick, 2000; Vincent, personal obs.). Males of all studied seahorse species were reproductively active almost continuously throughout the breeding season (Dauwe, 1993; Nijhoff, 1993; Vincent and Sadler, 1995; Perante et al., 2002), often remating the same day that they release their young (Vincent and Sadler, 1995). In our study, trawling may have occurred outside the primary breeding areas for male H. erectus, but catches of reproductively nonactive adult males during the breeding season also suggest that repeated trawling may have disrupted breeding in the population. A further indication of possible spatial structuring in the population (by reproductive status and size) is that almost half of the reproductively active males caught in 1998 were found in S2, the shallowest area; this area was not sampled in 1999 when few reproductively active males were found. Such spatial structuring offers the possibility of trawling outside the breeding area.
Indirect effects of the trawl fishery on mortality
Direct immediate mortality from trawling and culling was rather low, probably in part because trawl sets were of very short duration in this live-bait fishery. More importantly, most H. erectus caught in the Hernando Beach fishery were returned to sea, rather than retained as in some other Florida trawl fisheries (Vincent, pers. obs.). Indirect impacts of the fishery may, however, be considerable.
Seahorses caught in trawls may experience high postrelease mortality. A study in the live-bait shrimp trawl fishery in Tampa Bay, Florida (Meyer et al., 1999), found that only one of four of the congeneric dwarf seahorse (H. zosterae) caught as bycatch were alive in the holding tank of seawater 36 hours after collection (Meyerg). Such tows lasted only 5 minutes (Meyer et ah, 1999); therefore trawl-induced mortality could be greater in the Hernando Beach fishery (with trawls of 30-50 minutes), although H. erectus are larger and perhaps more robust than H. zosterae. Like other discarded bycatch, seahorses may also be subject to intense predation upon release. Predation on discarded fish has been observed on prawn trawlers in Australia (Hill and Wassenberg, 1990) and within the bait shrimp fishery of Tampa Bay (Meyer et al., 1999). Captains of bait-shrimp boats concurred that this is commonplace in the Hernando Beach fishery, and we frequently observed bottlenosed dolphins (Tursiops truncatus) and schools of lady fish (Elops saurus) swimming alongside the boats, feeding on discarded bycatch.
Trawling may significantly disrupt seahorse populations, particularly if they are spatially structured, as the present study suggests. The disproportionate removal of females could reduce mating opportunities, especially if H. erectus is monogamous, as are most studied seahorse species (e.g. Vincent, 1995; Vincent and Sadler, 1995; Kvarnemo et al., 2000; Perante et al., 2002). Trawling, on account of repeated intrusion onto breeding grounds, could also disrupt courtship and negatively affect reproduction, In heavily exploited areas of the fishery where fishermen repeatedly trawl productive areas, seahorses may face cumulative stress. For example, tail injuries are likely a serious wound for seahorses, given that their tails are essential to grasp holdfasts and may play a key role in mating competition, as they do with Hippocampus fuscus (Vincent, 1994).
Benthic habitat degradation is another potential indirect effect of live-bait shrimp trawling on seahorses. Bottom-fishing gear can reduce habitat complexity by removing emergent epifauna, smoothing sedimentary bedforms and by removing structure-forming species such as corals and sponges (Hutchings, 1990; Auster et al., 1996; Auster and Langton, 1999; Thrush and Dayton, 2002). Roller beam trawls also affect habitat complexity by redistributing macroalgae and seagrass (Meyer et el., 1999). We estimated that seagrasses comprised between 50% and 80% of the volume of the catch for each trawling operation. Although roller beam trawls are assumed to have low impact on seagrass habitat (Tabb and Kenny, 1969; Meyer et al., 1999), the effects of long-term repetitive trawling have not been tested, and it is possible that species composition and abundance, including that of H. erectus, have been adversely affected (Watling and Norse, 1998).
Summary
Despite the relatively low direct mortality of seahorse per boat, the live-bait trawl fishery has the potential to affect seahorse populations, both directly and indirectly. The key question is whether the level of exploitation, and subsequent impacts, represents a conservation concern. Our evidence is inconclusive. Perhaps only the skewed sex ratio and low proportion of reproductively active males suggest a potential problem. However, fishermen have consistently reported that seahorse catch per boat has declined greatly over the past two decades in this area. Effects of trawling are also almost certainly greater in food shrimp trawl fisheries, which trawl with much larger gear for longer periods, and obtain substantially more bycatch, with higher mortality. Our analysis should thus be seen as a first step in identifying areas for which more information is needed, specifically estimating abundance and fishing mortality, and understanding spatial structuring in H. erectus.
This paper focuses attention on the need for research on and monitoring of small fishes that may be affected by nonselective fishing gear. Management responses to minimize bycatch have focused primarily on seabirds, sea turtles, and commercially important finfishes, but trawl fisheries may also have significant impacts on the many small marine organisms obtained as bycatch, even if they comprise only a small proportion of the bycatch. Bycatch excluder devices are unlikely to be effective in reducing incidental catches of these species. Temporal variation in CPUE and spatial population structuring, as observed in our present study for H. erectus, suggest that time-area closures may be a pragmatic solution for reducing incidental catch.
Table 1
General linear model of effects of year, lunar phase, and area on the
CPUE (mm/(hxm) for nonzero H. erectus bycatch trawls in the Hernando
Beach bait-shrimp trawl fishery.
CPUE by trawl
(excluding
CPUE of
three highest
CPUE by trawl nights)
Source n F P n F P
Year 205 15.2 <0.0005 173 3.9 0.049
Lunar phase 205 173
cos([theta]) 19.1 <0.0005 4.1 0.045
sin([theta]) 8.9 0.003
1998 areas (I1, I2, N1,
N2, S1, S2) 116 7.4 <0.0005 105 2.3 0.037
1999 areas (N1, N2, N3,
N4) 87 8.3 <0.0005 68 0.3 0.85
Table 2
Descriptive statistics (sample size [n], mean, standard deviation
[SD], minimum [min] and maximum [max]) for standard length andweight of female (F) and male (M) seahorses. P values and sample
sizes (n) indicate results of t-tests evaluating sexual dimorphism
in SL and weight.
Sex n mean
Standard length (mm) M 201 146.1
P<0.0005, n=465 F 264 134.4
Weight (g) M 105 12.5
P<0.002, n=232 F 140 11.0
SD min max
Standard length (mm) 21.9 105.3 200.5
P<0.0005, n=465 20.1 105.3 202.0
Weight (g) 5.0 6.0 27.0
P<0.002, n=232 5.8 3.0 31.0
Table 3
Contingency tables on effects of year, lunar phase, and area
on population structure of seahorses in the bycatch of the
Hernando Beach bait shrimp trawl fishery. Yates corrections
were applied to 2×2 contingency tables.
Juveniles:Adults
Source [chi
df square] P
Year 1 2.46 >0.10
Lunar phase 3 11.00 <0.025
Area 3 8.84 <0.05
Sex ratio
Source
[chi
Year df square] P
Lunar phase 1 0.15 >0.50
Area 3 3.69 >0.25
7 13.3 >0.05
Reproductive state
Source [chi
df square] P
Year 1 27.89 <0.001
Lunar phase 3 7.80 >0.05
Area 2 21.64 <0.001
Acknowledgments
This paper is a contribution from Project Seahorse. We thank Jana Schulz for her assistance with fieldwork, Daniel and Patricia Mohr for their support during fieldwork, James Boxall for preparing the map, A. DeBruyn, L. Crowder, M. Kaiser, and an anonymous reviewer for providing helpful comments on an earlier draft of this manuscript. This study would not have been possible without the cooperation and support of many of the shrimp boat captains and crew in Hernando Beach. This research was funded through an NSERC summer undergraduate award to JKB, support from the Community Fund (UK) and Guylian Chocolates Belgium for JJM, and an NSERC operating grant to ACJV.
(1) Vincent, A. C. J.. K. L. Evans, and A. D. Marsden. 2003. Home range behaviour of the monogamous Australian seahorse, Hippocampus whitei. Manuscript in review.
(2) Vincent, A. C. J., and A. Perry (eds.). 2003. Global trade in seahorses. Manuscript in prep.
(3) Vincent, A. C. J., J. J. Meeuwig, M. G. Pajaro, and N. C. Perante. Seahorse catches in the central Philippines: characteristics and conservation implications. Manuscript in prep.
(4) Meeuwig, J. J., D. H. Hoang, T. S. Ky, S.D. Job, and A. C. J. Vincent. Bycatch landings of seahorses in central Vietnam. Manuscript in prep.
(5) Meeuwig, J. J. Life history parameters of the exploited seahorse Hippocampus comes: a length based analysis. Manuscript in prep.
(6) Curtis, J. 2002. Unpubl. data. Project Seahorse, Fisheries Center, The University of British Columbia, 2204 Main Mall, Vancouver, BC, V6T 1Z4, Canada.
(7) Martin-Smith, K. 2002. Unpubl. data. Project Seahorse, Fisheries Center, The University of British Columbia, 2204 Main Mall, Vancouver, BC, V6T 1Z4, Canada.
(8) Moreau, M-A., and A. C. J. Vincent. 2000. Unpubl. data. Project Seahorse, Fisheries Center, The University of British Columbia, 2204 Main Mall, Vancouver, BC, V6T 1Z4, Canada.
(9) Meyer, D. 1999. Personal commun. NOAA Center for Coastal Fisheries and Habitat Research, Beaufort Laboratory, 101 Pivers Island Road, Beaufort, NC 28516.
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Julia K. Baum
Jessica J. Meeuwig
Amanda C.J. Vincent
Project Seahorse
Department of Biology
McGill University,
1205 Dr. Penfield Ave.
Montreal, Quebec, H3A 1B1, Canada
Present address (for J. K. Baum): Department of Biology
Dalhousie University
1355 Oxford St. Halifax, Nova Scotia, B3H 4J1, Canada
E-mail address (for J. K. Baum): baum@mathstat.dal.ca
Manuscript approved for publication 2 June 2003 by Scientific Editor.
Manuscript received 26 June 2003 at NMFS Scientific Publications Office.
COPYRIGHT 2003 National Marine Fisheries Service
COPYRIGHT 2004 Gale Group