Study Area

This study focuses on the northwestern coast of BC, Mexico, from the United States (US)-Mexico border (32.5º N) to San Quintín (30º N) (Fig. 1). This coastal zone contains the highest human population density of the Baja California Peninsula, and high levels of infrastructure and coastal Access are present (^{INEGI 2019}). Coastal waters in this región are strongly influenced by the cold, southward-flowing California Current and are characterized by high primary productivity that is driven by upwelling (^{Zaytsev et al. 2003}, ^{Durazo 2009}). The coastal region is generally inhabited by marine species pertaining to the San Diego marine province (^{Horn et al. 2006}). The continental shelf is relatively narrow, extending 6-26 km offshore.

Figure 1 Location of the 4 artisanal fishery ports surveyed in this study along the northwestern coast of Baja California, Mexico. Dotted black lines delimit the fishing areas for each camp. The 200-m (inner) and 1,000-m (outer) bathymetry lines are shown in grey. CI = Coronado Islands; PB = Punta Baja; BSV = Bahía Sebastián Vizcaíno; LM = Laguna Manuela. The insert shows the location of the study area relative to the United States and mainland Mexico. 

Overview of data collection

There are 19 artisanal fishing camps within the study region (^{Cartamil et al. 2011}). In the present study, we concentrated our survey effort on 4 of the largest camps (Popotla, Ensenada, Erendira, and San Quintín; Fig. 1). We utilized data from various camp surveys conducted opportunistically over a 15-year period (1995-2010, Table 1). The first 2 Ensenada sampling events and the first San Quintín sampling event (Table 1) were carried out by graduate students from the CICESE Laboratory of Fisheries Ecology, while the remaining sampling events were carried out by staff from the same laboratory as part of a research project to characterize the capture of elasmobranchs in the region.

In the first visit to a sampling location, fishers indicated on a map the location of their fishing gears. Upon arrival of each vessel, the type of fishing gear and its characteristics were documented. When possible, the entire catch landed by a vessel was recorded, including bony fishes and invertebrates (although not reported in this study). Landed elasmobranchs were identified to the species level when possible using regional identification guides (^{Eschmeyer et al. 1983}). In most cases, sex was recorded by the presence or absence of male claspers.

Due to the different seasons across sampling dates and locations, the procedures used in this study were adapted from various publications (^{Bizzarro et al. 2009b}, ^{Smith et al. 2009}, ^{Cartamil et al. 2011}) and are described in the following section.

Artisanal fishing camp sampling

Popotla is the largest artisanal fishing camp near the US-Mexico border and has been used as a fishing port for over 40 years. Thirty-six sampling trips between October 2008 and October 2010 were conducted to evaluate fishing effort, species composition in catches, and total capture by artisanal fishing vessels (Table 1). All elasmobranch measurements were recorded to the nearest 0.5 cm. For shark species, total length (^{TL, Compagno 2001}) was recorded using the natural extension of the caudal fin, but fork length (FL) was used for thresher sharks (Alopias vulpinus). For batoids, disc width was recorded, but TL was used for Pseudobatos and Zapteryx guitarfishes. Smoothhound sharks (i.e., Mustelus californicus and Mustelus henlei and possibly Mustelus lunulatus and Mustelus albipinnis) were grouped into one category (Mustelus spp.) to avoid misidentification. In some cases, unambiguous identification was possible for M. henlei, and these data were reported separately.

Landing per unit effort (LPUE) was calculated per species for each major gear type, where the landings were taken to be the number of individuals landed per species and the unit of effort was a single fishing trip. However, fishing effort was not standardized given that vessels often fished with a variable or an unquantifiable number of hooks or nets.

The assumption of equal sex ratios (1:1) within the landings was tested using chi-square analysis with a Yate correction for continuity (^{Zar 1996}, ^{Bizzarro et al. 2009c}). This analysis was performed for elasmobranch species for which sex data were available and >40 individuals were landed. Histograms of sex-specific sizes were plotted for each elasmobranch species and interpreted using size-at-maturity data from ^{Ebert (2003)}. In cases where n < 10, only the summary statistics of mean size ± standard deviation (SD) were given. Similar analyses were conducted on data collected at the other artisanal camps.

The Ensenada artisanal fleet has used longlines to target pelagic elasmobranchs for over 30 years. We sampled landings by longline vessels during the periods from June 1995 to March 1996, July 1999 to May 2000 (^{Furlong-Estrada et al. 2017}), and 2007 to 2010 (Table 1). Given that sharks were beheaded and gutted at sea before returning to port, we measured the alternative length (AL, the distance from the origin of the first dorsal fin to the origin of the second dorsal fin) of each shark. For reporting purposes, AL was converted to TL (or FL for thresher sharks) using equations that were defined by the measurements made during this study and data collected by ^{Cartamil et al. (2011)} (Table 2). For cases in which dressed sharks (i.e., head and viscera removed) were brought into port with their corresponding fins, we calculated the weight of the fins (i.e., dorsal, pectoral, pelvic, and lower caudal lobe) as a percentage of the dressed weight. Additionally, fisher income from the sale of shark fins was quantified to ascertain the average ex-vessel fin price per kilogram.

At the Port of Erendira, we analyzed landing records from the sole fishing cooperative, Productos Marinos Erendira, from January 2006 through December 2008 (Table 1). The records consisted of the total weight of each fish species captured per day by each fishing vessel. In some cases, certain low-value species were recorded as species groups. Specifically, 3 species groups comprised of Myliobatis californica and Gymnura marmorata, Squalus suckleyi and Heterodontus francisci, and Pseudobatos productus and Zapterix exasperata, respectively, were created by the fishing cooperative because of their similar economic values. In addition, thresher shark length data were collected opportunistically during this period.

At San Quintín, 13 sampling trips were made to the fishing camp from August 2000 to May 2001 (Table 1). Blue shark individuals were measured and sexed, and the number of all other captured species was recorded.

Popotla

Fishing activity took place within an area spanning 30 km along the US-Mexico border and 20 km from the coast, including the waters around Coronado Islands (Fig. 1). Most of the product brought into this camp by fishers was sold directly to buyers on the beach. Landings from 194 artisanal fishing vessels were documented during 36 d of port sampling. The recorded fishing gears were bottom-set (18-60 m depth) gill nets that were 0.08-2.4 km in length with mesh sizes of 5-20 cm (stretched opening). Each documented vessel used several gill nets (up to 4) that were placed at various locations. Gill nets were checked every 24 h and were pulled out only when bad weather approached. The gill net fishing effort was primarily directed at teleosts, which were comprised of 23 species. In addition, important seasonal fishing activity was recorded for crabs (Cancer spp. and Stenocionops spp.), sea urchins (Strongylocentrotus franciscanus), and California red lobster (Panulirus interruptus), which constituted a significant portion of the landings (data not presented in this study).

Elasmobranchs comprised a significant portion of the incidental catch, with a total of 985 specimens from 19 species; in addition, one Holocephali species (29 specimens) was recorded (Table 3). With the exception of white sharks (Carcharodon carcharias) that were returned to the wáter because of their protection status (^{DOF 2007}) and thornback guitarfish (Platyrhinoidis triseriata) and spotted ratfish (Hydrolagus colliei) that were used only as bait for traps, all sharks and rays were marketed for human consumption, including a 6.64 m TL female basking shark (Cetorhinus maximus). The spiny dogfish shark (S. suckleyi) was the most commonly landed elasmobranch. This species comprised 21.99% of total landings and presented a F:M sex ratio of 7.2:1.0 (P < 0.001) and a LPUE value of 1.155. Other commonly landed elasmobranchs included the thornback guitarfish (P. triseriata; 17.36% of total landings, LPUE = 0.912), bat ray (M. californica; 16.37% of total landings, LPUE = 0.860), and Mustelus spp. (12.03% of total landings, LPUE = 0.632). For the spiny dogfish shark (F:M, 7.2:1.0; P < 0.001), a significantly greater number of females were captured tan males, whereas the opposite was observed for the thornback guitarfish (F:M, 1.0:68.0; P < 0.001) (Fig. 2).

Figure 2 Sex-specific size frequency distributions for chondrichthyans landed by the Popotla artisanal fishery. Sample size (n) refers to the number of measured individuals upon which the size histograms are based and not necessarily the total number captured. Females are depicted in black, males in white, and unknown sex in grey. 

Ensenada

The artisanal pelagic elasmobranch fishery based in Ensenada utilized an area within 50 km from the coast that extended 30 km to the north and south (Fig. 1). From June 1995 to March 1996, we documented a total of 697 blue sharks, 23 shortfin mako sharks, and 1 soupfin shark during 15 sampling days. Blue shark sizes ranged from 72 to 265 cm TL, although the sizes of most specimens ranged between 100 and 160 cm TL (Fig. 3a). During sampling trips from July 1999 to May 2000 (reported by ^{Furlong-Estrada et al. 2017}), 433 blue sharks were captured that ranged in size from 70 to 250 cm TL, with most individuals measuring between 90 and 130 cm TL (Fig. 3b).

Figure 3 Sex-specific size frequency distributions for Prionace glauca as built from data collected during the Ensenada and San Quintín longline sampling efforts. (a) Ensenada, 1995-1996. (b) Ensenada, 1999-2000 (modified from ^{Furlong-Estrada et al. 2017}). (c) San Quintín, 2000-2001. For each panel, sample size (n) refers to the number of measured individuals upon which the size histograms are based and not necessarily the total number captured. Females are depicted in black and males in white. 

During the more recent sampling dates from 2007 to 2010 (221 artisanal fishing vessels sampled over 74 d), we documented the use of longlines (3.0-4.5 km in length) with an average of 350 J-hooks (6-8 cm in length). Longlines were set near the surface to target pelagic elasmobranchs, and 5,231 individuals belonging to 8 species were examined (Table 2). The most abundant species was the blue shark (93.54% of total landings, LPUE = 22.241), followed by the shortfin mako shark (3.71% of total landings, LPUE = 0.878). Blue sharks ranged in size from 68 to 446 cm TL, although most sharks measured between 100 and 140 cm TL. Shortfin mako sharks ranged in size from 70 to 203 cm TL, although most sharks were between 90 and 150 cm TL (Fig. 4a). Taking into account known length-at-maturity data, most specimens were deemed immature. In addition, the F:M ratios were not significantly different from 1:1 for any of the surveyed species.

Figure 4 Length-frequency distributions for elasmobranchs landed by the Ensenada artisanal longline fishery (2007-2009) (a) and the Erendira artisanal fishery (2006-2008, Alopias vulpinus only) (b). Tabular data in (a) provide the summary statistics for Alopias superciliosus. Sample size (n) refers to the number of measured individuals upon which the size histograms are based and not necessarily at the total number captured. Sex was unknown for all specimens. 

During sampling (2007-2010), blue sharks constituted more than 80% of the landings throughout the year, with a maximum of 96% of the landings during summer. Shortfin mako sharks constituted between 1.4% and 4.3% of the landings throughout the year. Thresher sharks constituted between 2% and 3% of landings during the spring and summer seasons.

In contrast, soupfin sharks formed a very small percentage of landings year-round, although this species constituted 12% of landings during the winter season (Fig. 5). In general, the soupfin sharks were primarily large mature animals.

Figure 5 Seasonal differences in catch composition for the 4 most commonly landed elasmobranch species at Ensenada during the 2007-2010 sampling period. Shading indicates species: light gray, Prionace glauca; black, Isurus oxyrinchus; white, Alopias vulpinus; dark gray, Galeorhinus galeus. 

Of the 6,012 kg (dressed weight) of landed blue sharks, 320 kg were fresh fins. Thus, blue shark fins comprised 5.3% of the dressed weight. During this study, the ex-vessel Price for blue shark meat was $1.15 USD·kg-1 and the price of fresh fins was $13 USD·kg-1. Shark fins were marketed in 2 ways. The artisanal fishers delivered fins to the local cooperative and received 50% of the ex-vessel value or the fishers dried the fins and marketed them independently to obtain a higher profit.

Erendira

The artisanal fishing activity occurred principally in coastal waters, with occasional excursions of distances of up to 30 km offshore that extended 30 km to the north and south (Fig. 1). The Erendira fishing cooperative was comprised of 16 vessels that targeted teleosts and elasmobranchs. We analyzed fishing data from 2,580 fishing trips carried out over 540 d (an average of 8 fishing trips per day). The fishing methods used in these trips were bottom-set gill nets (1.0-1.5 km in length, depths of up to 5 m, and mesh sizes of 6-8 cm), which were pulled out by hand and checked every 24 h, and hook and line gears used to target rockfishes (Sebastes spp.).

Fishing effort was primarily directed at bony fishes, and we identified 8 species that accounted for 14,045 kg of the total landed catch. Elasmobranchs constituted a large part of the incidental catch, with 15 species accounting for 13,166 kg of the total landed catch (Table 4). In 2006, the most abundant elasmobranch species captured were thresher sharks (1,419 kg, LPUE = 1.189), California bat rays (M. californica) and California butterfly rays (G. marmorata; 1,272.5 kg, combined LPUE = 1.067), soupfin sharks (826.6 kg, LPUE = 0.693), California skates (Beringraja inornata; 778.9 kg, LPUE = 0.653), and Mustelus spp. (716.9 kg, LPUE = 0.601). In 2007, the most abundant elasmobranch species captured were thresher sharks (A. vulpinus; 2,615.1 kg, LPUE = 1.887), California bat rays and California butterfly rays (2,334 kg, LPUE = 1.684), California skates (721.6 kg, LPUE = 0.521), soupfin sharks (494.2 kg, LPUE = 0.357), and smoothhound sharks (Mustelus spp.; 469.2 kg, LPUE = 0.339). In addition, we collected data from 247 thresher sharks (n = 247, 59-183 cm FL) that were mostly juveniles, with 75% of the specimens presenting sizes of 80-110 cm FL (Fig. 4b).

San Quintín

During the sampling period (2000-2001), the artesanal fishers targeted pelagic sharks in waters 30-60 km offshore, with the fishing area extending approximately 50 km to the north and south (Fig. 1). Fishing gear consisted of pelagic longlines (3.0-4.5 km in length) that were set near the Surface and used 300-400 J-hooks (6-8 cm in length). However, this fishery no longer exists, and the remaining fishers use bottom-set gill nets sporadically or rods and reels. Currently, the fishery at this port is mainly directed towards invertebrates, such as sea urchins (S. franciscanus) and geoduck clams (Panopea globosa). From the 31 fishing trips sampled, we observed 1,463 blue shark specimens, 34 shortfin mako specimens, and 3 soupfin sharks. Blue sharks ranged in size from 60 to 220 cm TL (38% were between 100 and 120 cm TL) and their sex ratio was not significantly different from 1:1 (Fig. 3c).

Biological considerations

This study illustrates the diversity of elasmobranch fauna found in the coastal waters of northwestern BC. We documented 25 species of elasmobranchs in the landings of artisanal fisheries, which is similar to what has been registered for the southwestern coast of BC (^{Cartamil et al. 2011}) but differs from what has been reported for the northwestern coast of Baja California Sur, where 53 species of elasmobranchs have been documented (^{Ramírez-Amaro et al. 2013}). This result is worth noting given that both areas are adjacent yet divided by the physical barrier that is Punta Eugenia, which forms the southern part of Bahía Sebastián Vizcaíno (BSV).

Although species composition of elasmobranchs in northwestern BC was similar to that reported by ^{Cartamil et al. (2011)} for Laguna Manuela in BSV (located just south of the study area, Fig. 1), minor zoogeographical distinctions exist along the Pacific coast of BC. Specifically, ^{Cartamil et al. (2011)} found that the smooth hammerhead shark (Sphyrna zygaena) and the banded guitarfish (Z. exasperata) were commonly present in artisanal fishery landings in BSV. However, only one smooth hammerhead shark was observed in this study, and banded guitarfish landings were uncommon. Conversely, some species documented in the present study, such as the spotted ratfish and the basking shark, were not found in the southern region surveyed by ^{Cartamil et al. (2011)}. Spiny dogfish represented an abundant portion of the landings in the present study, especially at Popotla, although this species was only captured sporadically in the artisanal fishery of Laguna Manuela in BSV ^{(Cartamil et al. 2011)}. These minor differences in species composition could be due to dissimilarities between oceanographic characteristics in BSV and those in the present study area, which has lower average wáter temperatures, increased upwelling activity, a narrower continental shelf, and increased wind effects (^{Hernández-Rivas et al. 2000}, ^{Zaytsev et al. 2003}). Nonetheless, these oceanographic differences appear to affect only a few species that may be sensitive to small temperature changes (^{Horn and Allen 1978}). ^{Hopkins and Cech (1994}) indicated that all coastal elasmobranchs are virtually ectotherms and are thus susceptible to temperature changes. Specifically, ^{Hopkins and Cech (1994)} observed that the metabolism of the bat ray (M. californica) in Tomales Bay, California, was more efficient at higher temperatures (14-20 ºC) than at lower temperaturas and was thus more abundant during the summer months. Similar patterns can be observed throughout its distribution along the Pacific coast of Mexico and its abundance has been found to gradually increase from north to south (^{Cartamil et al. 2011}, ^{Ramirez-Amaro et al. 2013}) as temperature increases along the Baja California Peninsula.

Some sexual segregation was observed in the northernmost fishing camp, with females predominating in spiny dogfish (F:M, 7.2:1.0) and males in thornback guitarfish (F:M, 1.0:68.0) landings. Specifically, 60% of spiny dogfish specimens were adult females and the presence of embryos or capsules was noted upon dissection in several cases. About 80% of thornback guitarfish individuals presented disc widths of 20-30 cm. Additionally, almost all white shark (C. carcharias) and thresher shark specimens were juveniles. These results may be due to gill nets being set near the coastal zone, in what is likely an important birth and nursery habitat (^{Weng et al. 2007}, ^{Cartamil et al. 2010}, ²⁰¹⁶).

An increase in the landing rates of soupfin sharks was noted in the winter months in the Ensenada artisanal longline fishery. Similarly, ^{Cartamil et al. (2011)} found that most longline soupfin shark captures in BSV took place in Winter and that specimens that were captured offshore were primarily large mature animals. ^{Ramírez-Amaro et al. (2013)} evaluated soupfin shark landings along the Pacific coast of Baja California Sur, but these were primarily smaller animals that were captured during the summer months. These results suggest that size segregation is present in this species, with mature individuals primarily inhabiting the offshore waters of western BC during the winter months and smaller specimens being more abundant in nearshore waters during the summer months.

Dynamics of the fishing fleet

In Popotla, fishing was directed primarily at teleosts (23 species, 5,544 records, 84.5% of the total landings), which command a higher value than chondrichthyans (20 species, 1,017 records, 15.5% of the total landings). However, elasmobranchs constituted an important part of the landings, and in some cases the weight of elasmobranch landings surpassed that of bony fish catches. During the 2007-2010 sampling period in Ensenada, the catch was directed at elasmobranchs, which constituted nearly 100% of the records (8 elasmobranch species, 5,231 records; 4 bony fish species, 4 records). In Erendira, the catch was mainly directed at bony fishes (8 species, 90% of the landings); however, elasmobranchs were an important and specific component of the catch (15 species, 10% of the landings).

Differences in both the quantity and species composition of the landings varied substantially between the 4 surveyed camps, primarily due to the differences in fishing gear, sampling area, and time period. For example, bottom-set gill nets were the main fishing gear used at both Popotla and Erendira. However, fishing in Popotla was generally conducted close to shore, and in Erendira nets were placed further offshore and fishers also fished with handlines. At San Quintín and Ensenada, only surface longlines were used, which limited the catch to pelagic elasmobranchs.

Within the study region, we observed a higher concentration of fishing effort per unit area compared to that of the les populated BSV (^{Cartamil et al. 2011}). However, interviews indicated that most fishers had alternative forms of income, such as construction work. In addition, the proximity of the landing areas to major cities facilitated marketing, and products were sold either on the beach or at the local markets. Thus, BC fishery products generated added value due to their freshness, and the presence of fewer intermediaries between fishers and consumers generated more revenue with les effort and product.

In recent years, sharks have begun to be subjected to non-extractive exploitation via ecotourism on the Baja California Peninsula. For example, tourist activity in the form of white shark observations at Isla Guadalupe has doubled in just 5 years (OSM, unpublished data). However, due to the remoteness of the island (270 km offshore), the development of infrastructure to increase ecotourism activity is outside the possibilities of the artisanal fishers. In Bahía de los Ángeles, the fishing community is dedicated to ecotourism during the whale shark season (^{Cárdenas-Torres et al. 2007}). However, the fishing community in the study área has not yet explored this option because the species present in the area are not as charismatic or attractive as those off Guadalupe Island or in Bahía de los Ángeles.

Shark Fin Trade

This study documented the blue shark fin trade at the port of Ensenada. We observed that for every 100 kg of shark meat, 5.3 kg of fresh fins were processed, although this relationship may vary among species. This result agrees with previous findings reported by ^{Camhi et al. (2009)}, which also indicate a ratio of 5% shark fin to 95% shark meat. This ratio considers the dorsal fin, pectoral fins, and lower lobe of the caudal fin, although Ensenada traders also purchased pelvic fins. In any case, during the study period, fins generated income for fishers that was equivalent to that of the entire remaining carcass of the shark. However, due to the 2012 export ban on shark fins in California (^{OCEANA 2011}) and the decline in fin consumption in China, the shark fin price in the BC region decreased by almost 50%, resulting in an increase in meat value (pers. obs.).

On the west coast of BC, blue shark fins were considered to be of low quality and were sold for $28-34 USD·kg-1 during the study period, while the highest quality shark fins commanded a price of $40-50 USD·kg-1. Blue sharks are the most commonly landed species in the artisanal and industrial longline fisheries of the region (^{Sosa-Nishizaki et al. 2008}, ^{Cartamil et al. 2011}). Due to their ubiquity in open-ocean hábitats worldwide (^{Compagno et al. 1995}), blue sharks generate the highest income in shark fin markets worldwide (^{Clarke et al. 2004}), and the constant demand for blue shark fins is one of the main motivations for fishers to continue catching these sharks (^{Clarke et al. 2007}). Currently, blue shark fins have a value of $7-8 USD·kg-1, while the price of the highest quality shark fins is $30 USD·kg-1 (Cap. Miguel Chaidez, pers. comm.).

Management

Mexican fishery authorities are working in the conservation of all elasmobranch species that are subject to fishing pressure. Since May 2007, the NOM-029 regulation (^{DOF 2007}) has restricted fishing activity in order to protect elasmobranchs within coastal zones to allow these organisms to reach maturity. More recently, a ban on the directed fishing of elasmobranchs was instituted across Mexico, and this ban corresponds to the months from May through August in BC (^{DOF 2013}). However, if large differences in elasmobranch species composition exist between adjacent coastal regions, these differences may be important to consider in the design of region-specific fishery management plans. For example, ^{Castillo-Geniz et al. (2008)} recommended the division of the Mexican coastal regions into 6 separate regulatory zones for elasmobranch fishing. The recommended division was based on habitat, climate, fishery, species composition, and socio-economic characteristics.