A CONTEMPORARY PLAN FOR MANAGING WHITE SEABASS BROODSTOCK AND PRODUCTION COHORTS FOR THE OREHP

Transcription

1 A CONTEMPORARY PLAN FOR MANAGING WHITE SEABASS BROODSTOCK AND PRODUCTION COHORTS FOR THE OREHP ORIGINAL April 2008 REVISED January 2011 Prepared by Kristen Gruenthal, PhD Mark Drawbridge, MS Michael Gafford Hubbs-SeaWorld Research Institute 2595 Ingraham St. San Diego, CA USA Phone: Fax: HSWRI Revised WSB BPCMP December 2010 Page 1

2 This document was developed and revised by HSWRI scientists as a contemporary plan for managing white seabass (Atractoscion nobilis; WSB) broodstock and production cohorts in a practical manner that maximizes genetic diversity. This plan is intended to be adaptive and dynamic, taking advantage of the best, most recent information available. It also assumes that sufficient financial support exists to execute the plan, especially resources required to capture, transport, and hold new brood fish each year. BROODSTOCK MANAGEMENT Background and Contemporary Rationale The original WSB broodstock management plan implemented by HSWRI at the Leon Raymond Hubbard, Jr., Marine Fish Hatchery in Carlsbad, CA, was developed by Bartley et al. (1995). The plan involves four breeding pools that contain 50 adult fish of equal sex ratio in each pool. Each group of fish is conditioned to spawn for 4-5 month photothermally-regulated seasons that are offset from each other throughout the year. The initial plan recommended replacing 20% of the stock (10 fish per pool: five males and five females) and rotating 20% (five) of males from each pool into a different pool annually. During the past 15 years, new stock has been added inconsistently, primarily to replace fish that died or that were suffering from health problems, and replacement has averaged approximately 7% per year. Male fish have not been rotated among pools because of difficulties in handling fish, concerns of harming fish, and potential disruption of spawning cycles. A new plan was developed in early 2008, but the information contained in the document was based on genetic research that had not yet been completed. The contemporary management plan presented in this document seeks to correct and revise these inconsistencies and implement further improvements, including the development of: 1. revised fish replacement and sex ratio schemes 2. fish handling techniques for large brood fish in breeding pools 3. a routine collection and holding program to support introduction of new fish Each of these areas is covered in detail below. Replacement Scheme Outline 1. Census size (N): a. Bartley et al. (1995) i. 200 brood fish (50 fish per pool) 2

3 ii. For a minimum annual genetically effective breeding size (N b ) = 74 and assuming N b /N for hatchery ~0.5, then a minimum of 148 brood fish was required iii. Goal: N = 200 to include a conservative buffer of ~50 fish b. Gruenthal and Drawbridge (In prep) i brood fish (35-50 fish per pool) ii. For N b = 74 and N b /N = 0.64, then an absolute minimum of 116 brood fish is required iii. Revised goal: seek to maintain N = 200 brood fish as possible, but genetic quality assurance objectives can still be conservatively met with N = Sex ratio: a. Bartley et al. (1995) i. 50% female and 50% male ii. Based on assumed wild sex ratio of 50:50 iii. Goal: 50% female and 50% male b. Gruenthal and Drawbridge (In prep) i. 60% female and 40% male ii. Individual spawn events typically consist of contributions from 1-2 females and 6-7 males; females are limiting for genetic diversity iii. New ratio will increase female diversity, while not significantly impacting male contribution iv. Revised goal: 60:40 accounts for unequal contribution between sexes to spawning events and across spawning season 3. Replacement: a. Bartley et al. (1995) i. Goal: 20% replacement of each sex and 20% rotation of males among pools ii. Never fully implemented b. Gruenthal and Drawbridge (In prep) i. 25% replacement of existing brood fish annually with new fish from the wild ii. Assumes four-year generation time 1. males reach sexual maturity in 1-2 years and females in 2-3 years 2. all reproductively-mature fish assumed to have spawned at least once by four years of age iii. Revised goal: 25% replacement reduces the potential impact of year-to-year contributions from the same brood fish and maintains broodstock pool that is semi-representative of genetic variation in wild population even without pool rotation of males 1. replace 5-8 females and 3-5 males per year per pool, depending on census size 2. maintain sex 60:40 to the extent possible 3

4 Discussion We developed a revised fish replacement scheme in response to an internal assessment of current culture and management protocols and future needs. The initial plan of replacing 20% of the stock each year would result in fish remaining in the breeding program for five years. We are now recommending four-year residency time, recognizing that fish added to the breeding program may take 1-2 years to adapt and subsequently reproduce. We believe that this can be accomplished without significantly interrupting the existing breeding program, although the logistics of collecting and holding this many fish each year is significant. The new specific rotational procedure described below and illustrated in Figure ) A new sex ratio recommendation for brood fish of 60:40 female:male will be maintained in each of the four brood pools, to the fullest extent possible. 2) A minimum of 35 (20 females and 15 males; N = 140) to a maximum of 50 brood fish (30 females and 20 males; N = 200) will be maintained in each of the four pools. 3) Through this process, 25% of the fish will be removed from the program each year, resulting in a four-year residency time for each brood fish. As stated previously, the original broodstock management plan included rotating five males (20% per pool) from each breeding pool into a different pool (Bartley et al. 1995). However, by replacing more fish per year than specified in the original plan, we have attempted to mitigate the need to rotate males among tanks. The rotation of male fish among pools creates at least two potential problems. First, if males are moved at a time that is out of sync with the spawning season of the receiving pool, then there will be a potential disruption in spawning for that group. Second, since one group is always in a spawning mode, there will be a backlog in the rotation cycle caused by avoiding moving fish in or out of the group that is spawning. To compensate for this backlog would require additional fish holding capacity and extra handling of fish, a significant stressor. Hence, the rotation of males among brood pools is not a part of this contemporary broodstock management plan. Handling Techniques 1 Pending successful execution of an expanded collection and holding program, including funding to support it. Figure 1. Planned replacement scheme for brood fish. Diagram represents minimum N =

5 Handling techniques for individual brood fish have been well-established during collection efforts over the past 25 years. However, manipulating fish in high density brood pools at the hatchery provided some inherent challenges that had not been adequately addressed until recently. Specifically, the large number of fish and the hard fiberglass sidewalls represent hazards to the fish when they become startled. In recent years, we have gained valuable experience crowding and handling brood fish of other large species, including yellowfin tuna, California yellowtail and California halibut. The same general approach used for those species was applied to WSB, beginning in The primary difference was that WSB broodstock at the Carlsbad hatchery are maintained at four times the typical density of the other species mentioned in order to hold adequate numbers to maintain genetic diversity. During the initial handling sequence, the sex and identification via passive integrated transponder (PIT) tag of each fish was reconfirmed and new fin clip tissue samples were collected. In the future, we anticipate handling brood fish from each pool once each year, immediately following the spawning season for that group. The water level will be lowered and a vinyl crowder and sling will be used to move the fish (Figure 2). In addition to removing older stock from the population, this procedure provides us with the opportunity to examine fish, collect growth data, and obtain new genetic material, as needed. Collection and Holding Figure 2. Handling techniques used for WSB, including use of a vinyl crowder (top) and sling (bottom). A surplus number, if possible, of a WSB brood fish will be collected each year and maintained at HSWRI s net pen in Catalina Harbor at Santa Catalina Island, CA, assuming the facility can be maintained with adequate operational support. A surplus would ensure that adequate representation of each sex is available to satisfy the needs of sex ratio and replacement schedule. Adult fish will be captured from the wild using hook and line. Preference will be given to younger legal fish that are easier to handle. Sublegal fish (a.k.a. shorts ) may also be collected by individuals with scientific collecting permits. Shorts will need to be held and grown out for an additional 1-3 years after capture to reach sexual maturity. Broodstock collection efforts will be spearheaded by HSWRI in close cooperation with the United Anglers of Southern California (UASC), 5

6 the Sportfishing Association of California (SAC), and the California Department of Fish and Game (DFG). When needed for breeding purposes, sexually-mature broodstock will be transported to the mainland for a 45-day quarantine period at the Carlsbad Hatchery before introduction into the brood pools. 6

7 PRODUCTION RUN MANAGEMENT Background and Contemporary Rationale The production plan for WSB has historically involved year-round production of multiple cohorts. While numerous spawn events encompassing all four brood groups have been used, the production plan has been somewhat haphazard. Two extensive studies conducted in the past few years have placed us in a much better position to implement a more defined production plan leading toward a more predictable outcome. First, using parentage analyses, we have significantly improved our understanding of spawning patterns among our WSB brood groups, which in turn has improved our ability to maximize genetic diversity effectively within the practical considerations of a hatchery setting (Gruenthal et al. In review). Secondly, with a recently completed and very extensive mark-recapture modeling study, we have an improved understanding of survival rates of cultured seabass under different stocking scenarios that will help guide our production plan toward maximizing return rates (Hervas et al. 2010). Outline 1. Genetically effective population size arguments a. Assumptions i. Female equivalent (fe; Gruenthal and Drawbridge In prep) 1. Assume that one fe one effective female contributor (i.e. N f = 1) 3 L of eggs at 585 eggs/ml 2. For fe = N f = 1, the total effective number of hatchery breeders (N b ) = 3.12 ii. N b is assumed to be additive across spawning events b. Minimum annual N b = 74 required (Bartley et al. 1995) i. N b is independent of release limit ii. Goal: grow out fe s annually spread among the four breeding pools (Gruenthal and Drawbridge In prep) iii. Spawning events whereby fe > 1 are desirable to more easily meet goal from a production standpoint 2. Annual release limit a. Current limit: A sliding scale based on the number of broodstock as a proportion of the 200 target number, with a maximum quota of 350,000 fish released per calendar year i. Limit is reassessed every six months ii. Assuming 350K limit, ~12K juveniles released per fe, based on fe s per year iii. No genetic basis for sliding scale b. Genetically defensible quota: >1M annual release limit (Gruenthal and Drawbridge In prep) 3. Operational production considerations a. Releases are planned for all seasons except winter. 7

8 Discussion b. Release size is 20 cm minimum, with larger fish preferred c. Fish are raised in the hatchery to ~100 dph when they are tagged and ready for transport to outdoor raceways or cages, with cages being preferred d. When larval production is steady, growout areas become the potential bottleneck because of the volume requirements of larger fish; currently requires overwintering of fish to meet CY release targets We will begin maximizing the genetic diversity of the parental contributions within the annual release total to the fullest extent practical. The proposed operational model for the hatchery is to produce cohorts from female brood fish, independent of the release limit. Each cohort will be established using eggs from 1-4 spawning events occurring over a seven-day period. Fewer spawns will be used based on the number of fe s represented within a spawn (see below). Variability in stocking patterns is primarily due to egg availability. The seven-day period is dictated by the length of time during which sibling groups can be mixed with minimal effects from intraspecific aggression in the rearing pools. Spawn volume 2 also plays a role in the decision making process for cohort management. Spawn volumes are measured and used to quantify relative female contribution. We estimate that each female contributes an average of 1.8 million eggs (~3 L) at an average fecundity of 100 thousand eggs per kg body mass (Gruenthal et al. In review). That average value is considered one female equivalent (fe; Gruenthal and Drawbridge In prep). Spawn volumes during a given spawning event that are incrementally larger are considered to come from multiple. A female generally exhausts her store of eggs during a single spawning event (although contributions have been noted over 2-4 consecutive days), and the interval between spawns from individuals is close to a minimum of seven days (Gruenthal et al. In review). For simplicity, we assume that each spawn within a cohort is contributed by a different female because the eggs are collected over a short timeframe. Again, the goal is to annually grow out juveniles that are contributed to by individual females (24-32 fe s), partitioned among all four brood groups. Juvenile cohorts should also be divided as equally as possible within the release limit (e.g. for a quota of 350K, ~12,000 juveniles would be released per fe, depending on the actual number of fe s achieved for that year). References Bartley D.M., Kent D.B., and M.A. Drawbridge. (1995) Conservation of genetic diversity of white seabass enhancement program in southern California. American Fisheries Society Symposium, 15, A volumetric measure of the number of eggs. 8

9 Gruenthal K.M., Vetter R., and M.A. Drawbridge (In review). Genetic determination of broadcast spawning dynamics of a pelagic marine finfish in captivity. Can J Fish Aquat Sci. NOAA Awards NA06NMF and NA09NMF , OREHP Award P Gruenthal K.M., and M.A Drawbridge (In prep). Towards responsible stock enhancement: revising broodstock and juvenile production protocols in the California white seabass (Atractroscion nobilis) captive breeding program. Hervas, S., Lorenzen K., Shane M.A., and M.A. Drawbridge (2010). Quantitative assessment of a white seabass (Atractoscion nobilis) stock enhancement program in California: post-release dispersal, growth and survival. Fisheries Research. 105: NOAA Award NA06NMF