Arrecife Alacranes Research Articles

Evaluación del programa de conservación y manejo del Parque Nacional Arrecife Alacranes desde la perspectiva de la gestión de los recursos pesqueros

La evaluación de la efectividad en Áreas Naturales Protegidas representa un proceso de aprendizaje dirigido a reforzar el diseño y cumplimiento de las acciones comprometidas. Este artículo aborda el caso del Parque Nacional Arrecife Alacranes (PNAA) que cuenta con un Programa de Conservación y Manejo (PCyM) desde 2007 que hasta la fecha no ha sido evaluado ni actualizado. El objetivo de este artículo es evaluar la efectividad del PCyM en relación con la gestión de los recursos pesqueros en el PNAA e identificar los principales problemas administrativos. La evaluación abarca 54 acciones y diez años de operación del PNAA, a partir de fuentes primarias y secundarias en cuatro etapas. Los resultados señalan que el 55.6 % de las acciones son de la política de aprovechamiento. Se identificaron los puntos críticos del PNAA, en especial, la falta de continuidad de las acciones propuestas, lo que conlleva a la baja efectividad de la política de conservación directa. Asimismo, la reducción del 11 % del presupuesto total, es un obstáculo que la administración debe enfrentar en la gestión de los recursos pesqueros. Se concluye que los esfuerzos deben ser canalizados al Subprograma de manejo y restauración, ambos relacionados con la actividad pesquera y la gobernanza.

Perceptions of diver-fishermen and recreational divers on lionfish as a threat in the Parque Nacional Arrecife Alacranes, southern Gulf of Mexico

Worldwide, biological invasions remain an unrelenting environmental disaster. In the last years, the Indo-Pacific lionfish, Pterois volitans, has become one of the most highly surveyed biological invasions in Western Atlantic Ocean. This predator fish has the potential to affect the invaded ecosystem due to its voracity, fast population growth, and strong competition with native fauna for resources, which in turn implies a threat to coral reefs. A common strategy addressing this invasion relies on culling, which requires systematic volunteer's efforts on given areas of lionfish abundance to be effective. Understanding volunteer's perceptions towards this invasion is vital to engaging volunteers in culling campaigns. Here, we analyze perceptions, based on open questions, Likert-scale type, and Damage Schedule format, of volunteers (diver-fishermen and recreational divers), towards lionfish in the Parque Nacional Arrecife Alacranes (PNAA), off northern coast of the Yucatan Peninsula, Mexico. The results showed heterogeneous perceptions of volunteers shaped by distinct interests these groups have in the area, with diver-fishermen's concerns most related to their lobster catch and recreational divers referring mainly on uncontrolled tourism a major environmental problem in the PNAA. This scenario implies both groups remain unaware of the effects from this invasion on the coral reef ecosystem, but they showed willingness to participate in lionfish removals. A lack of robust awareness of these groups within a natural protected area in the southern Gulf of Mexico is of concern. We recommend further evaluations of users perspectives on lionfish as a threat and on lionfish as a viable market of consumption as strategy to control its population.

Discovery of Adults Linked to Cloning Oceanic Starfish Larvae (Oreaster, Asteroidea: Echinodermata).

Two juvenile specimens of a new species of Oreaster were collected at Parque Nacional Arrecife Alacranes and Triángulos Oeste in the southern Gulf of Mexico. DNA of mitochondrial loci identifies them as members of the same clade as cloning larvae of Oreaster found abundantly in waters of the Florida Current-Gulf Stream system, and distinct from Oreaster clavatus and Oreaster reticulatus, the two known Oreasteridae species in the North Atlantic. Larvae from the new species of Oreaster persist as clones but also metamorphose and settle to the benthos with typical asteroid morphology.

Chemical composition, amino acid and fatty acid profiles of Lionfish Pterois volitans from the Alacranes reef, southern Gulf of Mexico

The chemical and nutritional composition of fillet from Lionfish Pterois volitans L. captured in “Parque Nacional Arrecife Alacranes (PNNA)” in the Gulf of Mexico were investigated. The average chemical composition in dry weight basis of fillets was 25.41 % protein, 5.65 % ash, 0.82 % fat and 0.79 % fiber. The moisture in fresh fillet was 77.8 % on average. The fatty acids in greater proportion were SFA, followed by PUFA and MUFA, and with a w-3/w-6 ratio of 2.33. Docosahexaenoic acid was the most predominant (34.84 %). The most abundant amino acids were phenylalanine, glutamic acid, aspartic acid, isoleucine and lysine. Given its nutritionist profile, consumption of lionfish fillet could be a solution to control the invasive Lionfish population in the Gulf of Mexico.

Structure and composition of surgeonfish (Acanthuridae) and parrotfish (Labridae: Scarinae) assemblages in the south of the Parque Nacional Arrecife Alacranes, southern Gulf of Mexico

Herbivorous fish play an important role in structuring benthic communities by influencing the distribution and composition of algal assemblages in coral reef systems. The distribution of herbivorous fish is often driven by the interaction between the ecological and physiological capacities of the fish, as well as by physical attributes related to habitat characteristics. In Mexico, information on the condition of herbivorous fish populations is limited to fringing reefs in the Mexican Caribbean. This study evaluated the composition and structure of the surgeonfish and parrotfish at two depths on the leeward and windward sides of the reef in the south of the Parque Nacional Arrecife Alacranes, Southern Gulf of Mexico. Results revealed a robust and healthy herbivorous fish assemblage in terms of species richness, with high abundance and sizes up to 40% larger than the species reported in the Caribbean. The herbivorous fish abundance and biomass distribution were related to coral, bare substratum, coralline algae, turf, and rugosity. However, other physical or abiotic variables (e.g., wave intensity, sedimentation, and light penetration) could be playing an important complementary role of similar or greater importance, and could potentially influence the distribution of the herbivorous fish abundance and biomass. This study contributes to the knowledge of the composition and structure of the herbivorous fish assemblages in a reef system (Alacranes reef) with limited human access.

Occurrence of large thickets of Acropora prolifera (Scleractinia: Acroporidae) in the southern Gulf of Mexico

Acropora prolifera is a first-generation (F1) coral hybrid of the staghorn coral A. cervicornis and the elkhorn coral A. palmata. We observed large thickets of A. prolifera in the southern areas of the Parque Nacional Arrecife Alacranes (PNAA), off the northern Yucatan Peninsula, Mexico. Thickets were located at < 2 m depth, interspersed with seagrass Thalassia testudinum and A. cervicornis. This is the first record of large A. prolifera thickets in the southern Gulf of Mexico.

Optimizing confirmation of invasive species eradication with rapid eradication assessment

Summary Confirmation of invasive species eradication following management programmes is typically determined by waiting an arbitrary period of time before determining success or failure based upon the then obvious presence or absence of the target species. Rapid eradication assessment could be achieved more expediently by applying statistical models of the probability of detecting survivors and their offspring, using a grid of detection devices, for a given set of biological and monitoring parameters. We simulate estimation of the probability of eradication for invasive rodents on islands across a range of monitoring parameters in order to provide guidance to managers on the optimal values, diminishing returns and trade‐offs in monitoring to achieve a given level of confidence in successful eradication. We found that monitoring an island for survivors over 15–20 nights is optimal and that waiting longer than a year before commencing monitoring has a negligible impact on the estimated probability of success. The spacing between detection devices has a considerable influence on estimated probability of success but only when it is &lt;60 m. Intrinsic biological parameters of the target species have a substantial impact on confirming the probability of success, but validated field data are lacking to reliably incorporate these in current rapid eradication assessment models. We present case studies demonstrating cost savings of at least 5% to managers when applying rapid eradication assessment to the eradication of house mice Mus musculus from Isla Pájaros and Isla Muertos in Arrecife Alacranes (Scorpion Reef) Mexico, and implement the rapid eradication assessment model in an HTML GUI interface for eradication managers (www.rea.is). Synthesis and applications. Rapid eradication assessment (REA) is a powerful tool for managers to design optimal invasive species eradication monitoring programmes. We recommend island eradication managers routinely implement REA on small islands for the demonstrated cost savings and to accelerate eradication confirmation, ultimately facilitating island restoration.

Relations between fish length to weight, and otolith length and weight, of the lionfish Pterois volitans in the Parque Nacional Arrecife Alacranes, southern Gulf of Mexico

Described are the relations between fish length to weight, and otolith length and weight, of the lionfish (Pterois volitans) collected by diver-fishermen volunteers between 2011 and 2013 in a natural protected area (Parque Nacional Arrecife Alacranes) off the northern Yucatan Peninsula, Mexico. High correlations were detected between fish length and otolith length. Significant differences in the length-weight relations were found for males and females. Length of lionfish can be estimated from otolith length and weight. This study will be helpful in diet content studies of potential predators of lionfish helping to discern the size of lionfish consumed when otoliths are found in stomachs of predators.

Lionfish invaded the mesophotic coral ecosystem of the Parque Nacional Arrecife Alacranes, Southern Gulf of Mexico

After the first detection of the Indo-Pacific lionfish (Pterois volitans) off the Florida coast (USA) approximately 30 years ago, its populations have continued spreading throughout the Western Atlantic. The invasive lionfish is a threat to the marine ecosystems because individuals reproduce fast, reach high population densities, prey on many taxa without discrimination, disperse as larvae by currents, and lack predators (Cote et al. 2013). Now, the lionfish has been detected in the mesophotic coral ecosystems (Lesser and Slattery 2011), which are deep (30–150 m) fore-reef communities with low light adapted macroalgae and zooxanthellate corals and gorgonians (Lesser and Slattery 2011). These habitats are considered a buffer against anthropogenic disturbances that impact their shallow reef counterparts. It is necessary to document as much biological information as possible on lionfish not only to follow the progression of its invasion in the Western Atlantic, but also to identify locations where it is particularly abundant. Here, we report the presence of lionfish in the mesophotic coral ecosystem (>35 m deep) of the Alacranes reef (22°21′44, 22°35′12N; 89°36′30, 89°48′00W), the largest reef complex in the Southern Gulf of Mexico. Alacranes reef is located approximately 130 km off the northern coast of the Yucatan Peninsula, Mexico.While declared in 1994 a natural protected area known as Parque Nacional Arrecife Alacranes (PNAA), commercial and recreational fishing are allowed in the area. The PNAA is an important fishing ground for lobster (Panulirus argus) and groupers (Epinephelidae). During SCUBA incursions in a deep reef, known as BJuanchos^ (22°21′0.66′′N; 89°41′48.04′′W) located at southern PNAA, we sighted 30 lionfish (in ~100 m) on September 2014 and 40 (in ~100 m) on October 2015, at a deep range between 40 and 45 m (Fig. 1). These observations were conducted during SCUBA incursions—only one dive per year—at the reef BJuanchos^, which is a relatively large reef formation (~2 ha) comprising several high-relief boulders (5 and 10 feet over the bottom) containing corals, sponges, leafy algae, and coralline algae. Diving bottom time (less than 9 min per dive) at this deep site limited activities; thus, one diver counted lionfish while another diver speared individuals during each dive. Probabilities of recounting lionfish in each visit to BJuanchos^ could be minimized by the fact that the diving spot visited each year was not exactly the same as that in the previous years, but it was the same reef. A lionfish sample collected with a Hawaiian sling (Fig. 1) was taken to the laboratory for processing. Collected individuals showed a size range between 36 and 44 cm in total length (TL) (N = 12, September 2014) and 21 and 42 cm TL (N = 22, October 2015). In the Western Atlantic, the invasive lionfish has been reported in mesophotic coral ecosystems previously. In the Bahamas, individuals were found at 91 m depth (Lesser and Slattery 2011). During ROV video surveys, Nuttall et al. (2014) recorded almost 400 lionfish in banks of the northwestern Gulf of Mexico at a depth range of 50 to 176 m, with the greatest lionfish abundance between 80 m and 89 m. In the eastern Gulf of Mexico, trawl surveys revealed high abundance at depths between 40 and 80 m, with individuals up to 40 cm TL (Switzer et al. 2015). In 2010, the first record of lionfish in the Southern Gulf of Mexico was made at a mesophotic coral ecosystem (38 m deep) located 50 km west of the PNAA, but only two small individuals were sighted and one was collected (<15 cm TL) (Aguilar-Perera and Tuz-Sulub 2010). However, our present Communicated by O. A. Bergstad

The rainbow parrotfish (Scarus guacamaia) does not depend on mangroves as nursery habitats in the Parque Nacional Arrecife Alacranes, Southern Gulf of Mexico

The rainbow parrotfish (Scarus guacamaia) does not depend on mangroves as nursery habitats in the Parque Nacional Arrecife Alacranes, Southern Gulf of Mexico

Growth and mortality of red lionfish, Pterois volitans (Actinopterygii: Scorpaeniformes: Scorpaenidae), in the Parque Nacional Arrecife Alacranes, southern Gulf of Mexico, as determined by size-frequency analysis

Background. Biological invasions represent a threat to the ecological stability of the ecosystem. Two lionfish species, the red lionfish and the devil firefish—both tropical marine predators native to the Indo–Pacific—were introduced to the Atlantic Ocean. However, only one—the red lionfish, Pterois volitans (Linnaeus, 1758)—has reached the level of an invasive species so far. It is crucial to learn its biology and ecology to understand trends of its invasion and impacts on native fauna, and to determine ecological changes as the invasion progresses. This study reports the growth and mortality of P. volitans in coral reefs of the Parque Nacional Arrecife Alacranes, off the northern Yucatan Peninsula, Mexico, in Southern Gulf of Mexico. Materials and methods. Lobster diver fishers participated in voluntary captures of lionfish during the lobster fishing seasons of 2010–2012. In the laboratory, lionfish were taxonomically identified, measured, and weighed. Their length–weight relation was determined according to the commonly used equation: W = aTLb. Fish total lengths were used to calculate the parameters of the von Bertalanffy growth function (VBGF) of lionfish based on the size-frequency analysis routine ELEFAN in the FiSAT II software. Results. A total of 776 lionfish (9.0 to 38.9 cm in total length, 7 to 874 g in total wet weight) were collected. The VBGF parameters estimated were L∞ = 42 cm, K = 0.88–1, t0 = –0.107 years. The total mortality instantaneous rate (Z) calculated by the total length converted catch curve was 2.06 year–1. Conclusion. This study provides first calculation of growth parameters and mortality of P. volitans in Southern Gulf of Mexico as determined by size-frequency analysis. Results contribute to the knowledge on the biology of P. volitans and to a better understanding of aspects of its biological invasion in the region.

Growth and mortality of red lionfish, Pterois volitans (Actinopterygii: Scorpaeniformes: Scorpaenidae), in the Parque Nacional Arrecife Alacranes, southern Gulf of Mexico, as determined by size-frequency analysis

Growth and mortality of red lionfish, Pterois volitans (Actinopterygii: Scorpaeniformes: Scorpaenidae), in the Parque Nacional Arrecife Alacranes, southern Gulf of Mexico, as determined by size-frequency analysis

Spawning aggregations of three protogynous groupers in the southern Gulf of Mexico.

Spawning aggregations of red hind Epinephelus guttatus, tiger grouper Mycteroperca tigris and yellowfin grouper Mycteroperca venenosa were identified at two coral-reef systems: Arrecife Alacranes (emergent bank reef) and Bajos del Norte (submerged bank reef) on the continental shelf of the Yucatan Peninsula (Campeche Bank), Mexico. At both reefs, E. guttatus forms large spawning aggregations between February and March. At Bajos del Norte, M. tigris reproduces in a small, low-density aggregation in May, while M. venenosa aggregates at high densities for spawning between March and May. Multi-species use of an aggregation site by E. guttatus and M. venenosa was observed at Bajos del Norte. The identified spawning aggregations are apparently stable in location over time, and all three species were commonly observed to spawn within 1 week following the full moon. Development and survival of the larvae spawned in these aggregations are probably aided by a seasonal (spring-summer) upwelling in the north-east Campeche Bank. A permanent area closure at Bajos del Norte, currently outside any specific fisheries management area or regulations, would provide protection needed for the spawning aggregations of these three species.

Metazoan parasites of Mycteroperca bonaci (Epinephelidae) off the coast of Yucatán, Mexico, with a checklist of its parasites in the Gulf of Mexico and Caribbean region

Samples of black grouper, Mycteroperca bonaci, totalling 161 specimens, were collected at 3 protected natural areas in Yucatán, Mexico: Reserva de la Biosfera Ría Celestún (RBRC), Reserva Estatal de Dzilam (RED), and the Parque Nacional Arrecife Alacranes (PNAA). In this study, we present the results of a survey of the metazoan parasite fauna of these fishes, together with a checklist of the metazoan parasites recorded previously from black grouper in the Gulf of Mexico and the Caribbean Sea. Twenty five taxa were found in the survey (21 adults and 4 larval stages). Nineteen taxa represent new host records for M. bonaci, increasing the total number of parasite taxa recorded for this host species to 59 in the geographic area referred above. Differences in species composition and infection parameters of each group of parasites are presented and discussed.

New records of caridean shrimps (Decapoda: Caridea) from shallow water along the northern Yucatan peninsula coasts of México

The northern coast of the Mexican state of Yucatan has only been cursorily sampled in the past, with most of this effort concentrated on the largest coral reef in the Gulf of Mexico, Arrecife Alacranes. The present study reports on recent collections (June 2008 - September 2013) of caridean shrimp in Yucatan, including Arrecife Alacranes and other reefs, as well as coastal lagoons and beaches. Additionally, a number of species are reported based on older, unidentified museum material. Six species represent new records for the Gulf of Mexico [Janicea antiguensis (Chace, 1972), Gnathophyllum modestum Hay, 1917, Lysmata jundalini Rhyne, Calado and dos Santos, 2012, Periclimenes sandyi De Grave, 2009, Rapipontonia platalea (Holthuis, 1951a), Typton tortugae, McClendon, 1911], and 11 species are confirmed new records for Mexico [Lysmata ankeri Rhyne and Lin, 2006, L. pederseni Rhyne and Lin, 2006, L. rafa Rhyne and Anker, 2007, Ascidonia miserabilis (Holthuis, 1951b), A. quasipusilla (Chace, 1972), Neopontonides chacei Heard, 1986, Periclimenaeus maxillulidens (Schmitt, 1936), P. pearsei (Schmitt, 1932), P. schmitti Holthuis, 1951b, Typton prionurus Holthuis, 1951b, Processa manningi De Grave and Felder, 2012], adding up to a total of 17 newly reported species for the East coast of Mexico. Further, the colour pattern of several species is documented for the first time.

Mayan diver-fishers as citizen scientists: detection and monitoring of the invasive red lionfish in the Parque Nacional Arrecife Alacranes, southern Gulf of Mexico

Biological invasions research has increasingly incorporated the participation of citizen scientists to collect data for monitoring and management purposes. This study outlines collaborative efforts with Mayan lobster diver-fishers, who participated voluntarily as citizen scientists in surveys, to detect and monitor the invasive red lionfish, Pterois volitans, in a marine protected area off the northern Yucatan Peninsula, Mexico. We engaged these fishers by building local capacity and awareness of the threats of the lionfish invasion on the fishery resources of the area. During the lobster fishing season (July 2010–February 2011), 30 fishers collected 248 red lionfish (9–29 cm TL) and recorded ancillary data (day, month, depth, and approximate coordinates of capture site). This collaboration not only allowed the first lionfish detection in the Parque Nacional Arrecife Alacranes in 2010, but the volunteer-based monitoring effort revealed that the invasion reached levels from intermediate to advanced, with more than 200 specimens captured in less than a year. Our results support the notion that engaging citizen scientists can enhance research, reduce costs, and improve the possibility of a long term monitoring survey.

Length-weight and length-length relationships of the invasive red lionfish [Pterois volitans (Linnaeus, 1758): Scorpaenidae] in the Parque Nacional Arrecife Alacranes, Southern Gulf of Mexico

Summary Described are the length–weight (LW) and length–length (LL) relationships of the Indo–Pacific, red lionfish [Pterois volitans (Linnaeus, 1758): Scorpaenidae] – an invasive fish introduced into the coral reefs of the Western Atlantic. Volunteer diver-fishers speared a total of 455 red lionfish (range 9–35 cm TL) from July 2010 to December 2011 in the Parque Nacional Arrecife Alacranes, off the northern coast of the Yucatan Peninsula, Mexico. The LWR obtained in the combined data was W = 0.104TL3.309.

Opisthobranch Assemblages from a Coral Reef System: The Role of Habitat Type and Food Availability

The present study examines how the habitat type and food availability influence the small scale distribution and assemblages of opisthobranch species in the National Park Arrecife Alacranes, the largest coral reef system in the southern Gulf of Mexico. Samples were collected during December 2009, and April and September 2010 over different habitats (green, red, and brown algae, seagrass meadows, drifting Sargassum spp., rocks, corals, and sand). In total, 32 species were identified and a multidimensional scaling (MDS) analysis based on the Bray-Curtis similarity index was used to determine the main opisthobranch assemblages. The degree of association between the habitat type and the species composition was tested by a chi-square test and the significance of its value was determined by Monte Carlo randomizations. MDS results defined four faunal associations (algal, Thalassia, Sargassum, and rocky assemblages), which showed a significant relationship between the composition of opisthobranch species and the habitat type. These results suggest that food supplies and feeding strategies play a dominant role in the coexistence of species and their small-scale spatial distribution within the reef. The algal assemblage, the group with the highest species richness, was mainly composed of herbivores inhabiting several types of benthic algal communities, whereas the Sargassum assemblage, was a less diversified group comprised of carnivores feeding on epibiont hydroids and was well adapted to the pelagic environment. The distribution of the opisthobranchs over the different habitats largely reflected the variety and availability of their food items.

Relative Growth of the Land Hermit Crab, Coenobita Clypeatus (Anomura, Coenobitidae) from a Coral Reef Island, Southern Gulf of Mexico

Information on growth in hermit crabs is very scarce, possibly due to the inherent difficulties of extracting them from their shells. In this work we document the relative growth, sexual size dimorphism, and level of chelae asymmetry of the land hermit crab Coenobita clypeatus (Fabricius, 1787) collected on a coral reef island in the National Park Arrecife Alacranes, southern Gulf of Mexico, during December 2009 and September 2010. Specimens examined were anaesthetized in a clove oil-water solution to avoid unnecessary crab mortality. A total of 387 individuals was measured on six external variables: cephalothorax length and width, and both chelae in length and width. Taking the cephalothorax length as the reference dimension, the remaining measurements were fitted to a power function to examine the degree of allometry in growth and make comparisons between sexes. For each sex, an asymmetry index (AI) was calculated to determine the degree of heterochely, and the significance of its value was evaluated by means of null models. Numerically, males dominated the crab population in the largest size classes (&gt;12.6 mm CL) in both seasons. As in most anomurans, C. clypeatus males attained larger body sizes than females, but their relative growth was most similar. For both sexes, the growth of chela width was positively allometric (t-test, p &lt; 0.05). Also, AI values showed a heterocheleous condition, with the left chela significantly higher (p &lt; 0.001) than the right. Regarding our own results and previous reports, we propose that heterochely could be triggered during the glaucothoal and juvenile stages. Numerical dominance of males in the larger size classes is probably due to the segregation of ovigerous females from the main crab group, searching suitable areas for egg deposition, but also to a better condition of males to carry heavier shells than females, which in their turn have to invest a considerable amount of energy in reproduction.

Geographical variation in the genetic diversity and composition of the endangered Queen Conch Strombus gigas (Mesogastropoda: Strombidae) from Yucatán, México

In Mexico and elsewhere in the Caribbean, the queen conch Strombus gigas is an endangered species. Understanding the genetic connectivity of their populations will support management strategies for long-term conservation of the species. Genetic diversity and population differentiation was assessed from samples collected at Banco Chinchorro and Isla Cozumel in the Mexican Caribbean and at Arrecife Alacranes in the Gulf of Mexico. Samples were obtained from the commercial capture at Banco Chinchorro (n = 50) and Isla Cozumel (n = 40) on March 2004. On November 2004, a non-invasive method for the Arrecife Alacranes sampling was applied, taking the hemolymph of live animals (n = 65) and releasing them to the wild. The mitochondrial DNA variation at two genes (COI and Cyt-b) was analyzed. Genetic diversity at the three locations ranged between 0.55-0.65 in COI and 0.87-0.94 in Cyt-b, showing no bottleneck evidences. A non-significant fixation index (F(ST) = 0.019, p = 0.161) and a Maximum Parsimony Network tree that did not show particular clades associated with any of the geographical locations, suggested a lack of statistically significant genetic differentiation among populations. Nevertheless, the cline patterns observed in both genetic diversity and haplotypic frequencies from Banco Chinchorro through Arrecife Alacranes, and the larger genetic distance between these locations from those between Isla Cozumel, Banco Chinchorro and Arrecife Alacranes, suggest the possibility of a pattern of isolation-by distance. The role of the main current systems over the potential genetic differences in S. gigas populations along the Mexican Caribbean, and the conservation management of S. gigas at these locations as discrete units is discussed.