6 Life History Strategies of Batoids Michael G. Frisk Contents 6.1 Introduction ........................................................................................................................283 6.2 Demographics ....................................................................................................................284 6.2.1 Definition of Vital Rates ........................................................................................284 6.2.2 Rajiformes: Rajidae (Skates), Rhinobatidae (Guitarfish) ...................................284 6.2.2.1 Rajidae: Skates .........................................................................................284 6.2.2.2 Rhinobatidae: Guitarfish ........................................................................285 6.2.3 Myliobatiformes .....................................................................................................290 6.2.4 Torpediniformes: Hypnidae, Narcinidae (Electric Rays); Narkidae (Sleeper Rays); Torpedinidae (Electric Rays) ...................................................... 291 6.2.5 Pristiformes: Pristidae (Sawfishes) ...................................................................... 291 6.3 Comparative Life History Analyses ................................................................................ 291 6.4 Geographic Variation in Life History Traits .................................................................. 295 6.5 Compensatory Dynamics ................................................................................................. 296 6.6 Spatial Strategies ................................................................................................................299 6.6.1 Migration Patterns of Western Atlantic Skates ..................................................300 6.6.2 Migration and Ecosystem Management .............................................................304 6.6.3 Winter Skate and Georges Bank ..........................................................................304 6.6.4 Cownose Rays ........................................................................................................305 6.7 Conclusions .........................................................................................................................307 References .....................................................................................................................................307 6.1 Introduction Extant batoid species are grouped into 20 families and six orders that contain at least 513 species (McEachran and Dunn 1998; McEachran and Fechhelm 1998). The Rajidae is the most speciose family, with at least 232 species. Most batoids are benthic, feeding on crus- taceans, mollusks, polychaetes, and fishes (McEachran and Dunn 1998; Ebert and Bizzarro 2007; Mabragana and Giberto 2007). Batoids have adapted to a wide range of habitats, occur- ring in all the oceans, and they are commonly found in shallow estuarine, coastal, and shelf regions and in depths up to 3000 m (McEachran and Fechhelm 1998; McEachran and Aschliman 2004). Although most batoids are marine, the family Potamotrygonidae and some species in the family Pristidae occur in fresh water (McEachran and Fechhelm 1998). Interest in batoid fishes has grown and is exemplified by developing fisheries (NEFSC 1999), population collapses (Brander 1981; McPhie and Campana 2009), and interactions 283 © 2010 by Taylor and Francis Group, LLC 284 Sharks and Their Relatives II: Biodiversity, Adaptive Physiology, and Conservation with commercially important species (Brander 1981; Link, Garrison, and Almeida 2002; Myers et al. 2007). More broadly, the importance of batoids has been highlighted by their possible role in structuring ecosystem dynamics (Murawski 1991; Link, Garrison, and Almeida 2002; Heithaus 2004; Frisk et al. 2008). The comparative life histories of batoids have received less attention than teleost and other elasmobranch species. Here I will review life histories of batoid orders, and when possible provide life history analyses to draw comparisons with other phylogenetic groups. The aim of the chapter is to review the following subjects: (1) the current understanding of batoid life histories in relation to other phylogenetic groups; (2) the potential for com- pensatory population dynamics; and (3) migration patterns. Several batoid orders suffer from a paucity of published data and for those groups only a brief biological description is provided. One consequence of the taxonomic distribution of published data is that most of our understanding of the biology, ecology, and population dynamics of the cohort is derived from a single family, the Rajidae. 6.2 Demographics Data on age at maturity (Tmat), longevity (Tmax), and von Bertalanffy growth coefficients k( ) were obtained from existing comparative analyses and databases including the following: (1) Frisk, Miller, and Fogarty (2001); (2) Ebert, Compagno, and Cowley (2008); (3) Cailliet and Goldman (2004); and (4) the Pacific shark database (web page: psrc.mlml.calstate.edu/ current-research/shark-fisheries-database/). Additionally, searches were conducted in ISI Web of Knowledge for the years 2000 to 2008 with the following search terms: Batoid(s); Rajiform(es); skate(s); ray(s), sawfish(es), guitarfish(es) and all family names. The grey lit- erature was not specifically searched, but was included if it was used in previous peer- reviewed research. 6.2.1 Definition of Vital Rates I followed standard definitions of vital rates, including the following: (1) maximum size (Lmax) is based on observations of maximum total length for a species; (2) length at matu- rity (Lmat) is generally estimated at the point when 50% of individuals in a species or stock reach maturity; (3) longevity (Tmax) is the maximum age observed for a species or stock within a species; (4) age at maturity (Tmat) is estimated as the age when 50% of individu- als in a species or stock reach maturity; and (5) growth rate (k) is the von Bertalanffy growth coefficient. Theoretical estimates of maximum size or longevity based on analysis of growth parameters were not used in life history comparisons unless specifically identi- fied. For all analyses, appropriate units will be reported. 6.2.2 Rajiformes: Rajidae (Skates), Rhinobatidae (Guitarfish) 6.2.2.1 Rajidae: Skates Skates are bottom-dwelling marine species that feed on various demersal prey, including polychaetes, amphipods, decapods, and fishes (Richards, Merriman, and Calhoun 1963; McHugh 2001; Gedamke 2006; Ebert and Bizzarro 2007). For marine taxa, they are one © 2010 by Taylor and Francis Group, LLC Life History Strategies of Batoids 285 of the most vulnerable to population declines and extirpations as a result of exploitation (Dulvy et al. 2000; Dulvy and Reynolds 2002). Longevity and maximum size: Direct observations of longevities for Rajidae species over- all were as high as 20 to 28 years (Table 6.1). Where sex-specific estimates were provided females live an average of 17.5 (SD = 5.7) years, while males live on average 17.3 (SD = 4.94) years (Table 6.2). Individual estimates for the family range from 8 years for male Roundel skate, Raja texana (Sulikowski et al. 2007b), to 50 years for the common skate, Dipturus batis, in the Celtic Sea (DuBuit 1972). However, the latter estimate was based on extrapo- lation of von Bertalanffy growth parameters from the oldest aged specimen (23 years). Maximum total lengths in Rajidae range from 31 cm for the Argentinean zipper sand skate, Psammobatis extent, to the Chilean rough skin skate, Dipturus trachyderma, which reaches a theoretical maximum size of 265 cm (Licandeo, Cerna, and Cespedes 2007). Age and size of maturation: Based on a handful of estimates, females mature at an average age of 10.1 (SD = 3.26) years, about 64% of maximum age. Males mature at an average of 9.1 (SD = 3.30) years, approximately 60% of maximum age (Table 6.2). Length at maturation in skates ranges between 60% and 90% of maximum length (Holden 1973, 1974; Frisk, Miller, and Fogarty 2001; Ebert, Compagno, and Cowley 2008; Ebert, Smith, and Cailliet 2008). Growth: Rajidae are slow-growing fishes. Estimates of the von Bertalanffy growth coef- ficient k, which expresses the rate at which individuals achieve maximum size, ranges from very slow in the cuckoo ray, Leucoraja naevus, with k = 0.019 yr–(1) to relatively fast k = 0.50 yr–1 in the brown ray, Raja miraletus (Zeiner and Wolf 1993). Average growth rates for Rajidae were (kmale = 0.14, SD = 0.10; kfemale 0.12, SD = 0.07). Reproduction: All skate species are oviparous and produce egg cases. On average, they have higher fecundity than other elasmobranchs (Dulvy et al. 2000), with annual fecun- dity ranging from 17 in thorny skate, Amblyraja radiate, to 153 in thornback ray, Raja clavata (Holden 1972; Ryland and Ajayi 1984). Generally, fecundity is thought to range from 40 to 150 eggs per year (Hoenig and Gruber 1990; Ellis and Shackley 1995; Musick and Ellis 2005). However, estimating fecundity in skates is difficult because egg-laying rates in the wild are difficult to measure and fecundity is often reported as ovarian fecundity. The Dolfinarium Harderwijk’s aquarium “Ray Reef” exhibit contained five species of skate from 1993 to 2001 and provided an opportunity to observe egg production in captivity (Koop 2005). Over this period, 41,772 eggs were produced, of which 39% contained yolk sacs, and only 3% hatched. All species combined resulted in an annual average fecundity of 117 eggs