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Previous research has suggested that backcross hybrid striped bass (BX: sunshine bass female x striped bass male) perform as well as F 1 hybrid striped bass (sunshine bass: white bass female x striped bass male) for many economically important traits. We conducted trials to compare rearing and growth traits of selected backcross hybrid striped bass with hybrid striped bass (HSB). We spawned and reared larval BX fry in two hatchery cycles, measured fry growth and performance, and compared them to past performance of HSB fry. We conducted a grow-out trial of commercially-available hybrid striped bass versus backcross hybrid striped bass in replicated tanks to phase II fingerling size (approximately lOOg). Compared to HSB, in the hatchery phase, backcross hybrid striped bass exhibited lower fertilization rate, comparable swimbladder inflation rate, shorter time-to-weaning, lower survival, comparable growth, and comparable condition factor. After 60 days, HSB exhibited significantly better growth, survival, and feed conversion efficiency than BX. However, BX may have better potential market acceptance by virtue of having a lower condition factor, appearing longer and less deep-bodied than HSB. INTRODUCTION Striped bass (Morone saxatilis) and its hybrids constitute a major sector of aquaculture in the United States, with production of 10.5 million pounds in 2002 (Carlberg and Van Olst 2003). Commercially significant quantities of Morone stocks also are produced in Taiwan, China, and Israel. Approximately 43% of Morone production occurs in tanks (Carlberg and Van Olst 2003), much of that in recirculating aquaculture systems (RAS). Private and government marketing experts estimate that U.S. production could increase to 50 million pounds and $100 million in annual farm-gate revenue if market prices could be decreased only moderately (Halbrendt et al. 1991). Production costs for HSB could be reduced by 12% if selective breeding can yield 20% faster growth rates (Losordo and Westerman 1994). The majority of Marone production is not of striped bass, but of its hybrids with white bass (M. chrysops) because of their hardiness, rapid growth rate, and limited availability of female striped bass broodstock (reviewed by Harrell 1997). Further, female WB are considered easier and more predictable to strip-spawn. Hence, most commercial production 44 International Journal of Recirculating Aquaculture, Volume 5, June 2004 Performance of Sunshine Bass and Backcross Hybrid Striped Bass is of sunshine bass, the progeny of the white bass female x striped bass male interspecific cross. Research has also been undertaken to evaluate the performance of second generation hybrids and backcrosses. Several studies have suggested that the progeny of F 1 hybrid striped bass females backcrossed to striped bass males have merit for commercial production. Jenkins et al. (1998) compared the performance of backcross hybrids to F1 hybrid striped bass and striped bass to market size in a recirculating system. At harvest, no differences in growth rate or feed conversion were detected; specific growth of BX was intermediate to those of the other stocks, with survival lower. Tomasso et al. (1999) compared performance of F 1 and backcross hybrids between 20 and 90 grams for 56 days in tanks (Tomasso et al. 1999), and observed similar survival, growth, and feed conversion rates among the hybrid types. Detailed comparison of the hatchery performance of sunshine and BX hybrids is lacking. We hypothesized that use of selected hybrid striped bass and striped bass for the backcross might yield a high-performance BX hybrid. Should high performance be realized, progressive fish farmers might utilize this approach by selecting the best F1 hybrid females from their own production stocks, making the backcross by strip-spawning them and fertilizing the eggs with purchased milt from selectively bred striped bass males (Kerby 1983), thereby obtaining a high-performance production stock that, because of selection on the maternal stock, is suited to their particular farm conditions. In this study, we evaluated the production potential of backcross (BX) hybrid striped bass (female sunshine bass x male striped bass). We compared the hatchery and grow-out performance of BX spawned from select broodstock and sunshine bass (HSB) of common genetic background. Specifically, we evaluated spawning success, hatch rate, growth, survival, and feed conversion efficiency through the first six months of life. In order to benchmark our results, we ran a trial in replicated tanks with HSB supplied by the most widely-used commercial source of fingerlings (Keo Fish Farms, Keo, AR, USA) and compared their performance with our BX. All fish were cultured within intensive, controlled-environment, recirculating aquaculture systems. International Journal of Recirculating Aquaculture, Volume 5, June 2004 45 Performance of Sunshine Bass and Backcross Hybrid Striped Bass MATERIALS AND METHODS In July 2001, we crossed 4 hybrid striped bass females to each of 4 striped bass males to produce 16 families of backcross hybrid striped bass. Poor fertilization rates resulted in a low yield of fry from this spawn. However, there were enough fry to pool and conduct a pilot commercialscale hatchery run. Randomly-chosen samples from the resulting 4,000 fingerlings were used for the grow-out trial. In October 2001, we spawned 3 female HSB with 2 male SB and eventually produced 7,000 fingerlings. While this was not enough to grow in commercial-scale systems, we collected hatchery performance data to corroborate our July/August experience with BX. Fertilized eggs were incubated in McDonald jars. Upon hatch, the larvae swam out of the hatching jar and into a 1,900-L larval rearing tank. Replicate larval rearing tanks were stocked with approximately 25 larvae per liter. Tanks were maintained with recirculating water and surface oil-skimming pads for the first 5 to 7 days post-hatch until swim-bladder inflation was completed. For the second week posthatch, larval rearing tanks were maintained static with gentle central aeration, except for an exchange of about 30% of the tank water over the course of two hours each day. During the second week post-hatch, larvae were fed rotifers enriched with Algamac (Aquafauna BioMarine Inc., Hawthorne, CA, USA) maintained at a density of 10/ml. By the end of the second week, larvae were weaned from rotifers to Artemia nauplii. By the end of the third week, the larvae were typically weaned from enriched Artemia to artificial feed (Biokyowa Inc. Girardeau, MO, USA). Weaned larvae were graded just once around 60 days post-hatch to remove the top 1 % to 2% of larger fish responsible for cannibalism. In November 2001, we purchased lOg fingerlings that represent typical, industry-standard HSB from Keo Fish Farms (Keo, AR, USA). These HSB closely matched the size and weight of our BX from the July spawn for the grow-out trial. BX and HSB fingerlings were each stocked separately in triplicate 1,900-L round tanks connected to the same recirculating system and reared at 23°C and a salinity of 1-3 ppt. All six tanks were similarly stocked initially (approximately 1 kg/m3). Fish were offered 80% of their expected ration via belt-feeders, and handfed to satiation twice a day. Approximately every 3 weeks, length and weight samples were taken from a randomly-chosen 15% of each tank's 46 International Journal of Recirculating Aquaculture, Volume 5, June 2004 Performance of Sunshine Bass and Backcross Hybrid Striped Bass population. At the conclusion of the project, the tanks reached typical commercial densities of 5 to 9 kg/m3• We quantified reproductive and hatchery performance of the three groups, including fertilization rate, swimbladder inflation rate, days to weaning onto artificial feed (i.e., date of first ingestion to date that >90% ingested artificial feed), survival estimates to approximately 60 days after hatch and to the end of the study, feed conversion rate, and monthly average weight and length. The trial continued until mid-February 2002 when the HSB averaged approximately lOOg. Results from previous years' hatchery efforts with HSB using similar rearing protocols were used for comparative purposes. Both the hatchery and grow-out systems employed water recirculation technology including drum-filters, fluidized-bed biofilters, oxygenation/ ozonation, and automated pH and temperature control that maintained high water-quality standards throughout the trials. Differences in fish weight between the hybrid types were analyzed as a repeated-measures general linear mixed model. Residual (restricted) maximum likelihood was used to estimate the model parameters. Initial fish biomass in each of the tanks and the amount of feed provided to each of the tanks were included in the model as covariates to assess the influence of these concomitant variables on differences in weight. A firstorder autoregressive covariance structure was assumed for the repeatedmeasurements of fish weight. Differences in fish survival at the end of the grow-out trial were analyzed as a general linear model with tanks serving as a blocking factor. Because fish survival/mortality is a binary response and thus will violate an assumption of normality, the significance of the test comparing survival between the hybrid types was obtained by randomization (number of randomizations= 1,000), whereby the F-statistic of the original test was compared to F-statistics obtained by randomly allocating the survival/ mortality data to the different hybrid types. The Type-I error rate for all statistical tests was set equal to 0.05. International Journal of Recirculating Aquaculture, Volume 5, June 2004 47 Performance of Sunshine Bass and Backcross Hybrid Striped Bass Table 1. Comparison of hatchery traits of backcross (BX) and Fl hybrid (HSB) striped bass for traits of economic interest. Traits of interest BX-HSB BX-HSB HSB HSB HSB

Hatchery and Grow-out Performance of Sunshine Bass and Backcross Hybrid Striped Bass in Recirculating Aquaculture Systems