Size specific fecundity of red abalone : evidence for reproductive senescence?

Journal of Shellfisheries Research, August, 2004 by Laura Rogers-Bennett, Richard F. Dondanville, Jerry Kashiwada

ABSTRACT The fecundity of wild red abalone. Haliotis rufescens, was examined during four reproductive seasons (2000-2003) in northern California. A broad size range of abalone were sampled (n = 425) from Van Damme State Park and the Point Arena area. Sexual maturity was defined as the presence of sperm or mature oocytes 170-190 [micro]m in diameter, with a jelly coat, detached from the trabeculae in the gonad. Histologic examination revealed that abalone <50 mm in shell length had not yet sexually differentiated and that 50% of the females from 105-130 mm and the males 75-95 mm had mature gametes while all larger animals were mature. Fecundity, as measured by an estimate of the number of mature eggs per female (X), increased exponentially with increasing shell length (Y) until the peak at 215 mm in shell length after which mature egg number began to decline. The largest female 260 mm (10.24 inches) had >80% necrotic eggs. The data were fit to a non linear Gaussian curve with 3 parameters;

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.]

the maximum productivity (A = 2,850,000 eggs [y.sup.-1]), size at maximum productivity ([micro] = 215 mm), and standard deviation ([sigma] = 38 mm) of the distribution of maximum productivity versus size. We conclude that whereas large females in excess of 215 mm in shell length undergo some senescence (decline in egg production), these females could potentially contribute as much to reproduction as the mid-size (130-215 mm) females. This suggests that management strategies that protect large females such as marine protected areas or de facto reserves will help maintain egg production and that more work is needed to better understand the relationship between female size and egg necrosis.

KEY WORDS: abalone reproduction, Haliotis rufescens, size at maturity, necrotic eggs, molluscs, northern California

INTRODUCTION

A basic understanding of reproduction and other vital rates for red abalone, Haliotis rufescens (Swainson 1822), populations is necessary for both ecologic studies and fishery management of wild populations. Management strategies such as minimum legal sizes depend on reproduction occurring prior to the onset of fishing. Knowledge of size specific fecundity and size at maturity can be used in size-structured models to examine the population dynamics of wild stocks. Whether the largest oldest females contribute to reproduction, undergo partial or complete reproductive senescence has important implications for population dynamics and management.

Red abalone populations in northern California support an important recreational fishery. Unlike the southern abalone fishery, which was closed in 1996 following intensive fishing (Dugan & Davis 1993, Karpov et. al 2000), the northern fishery is very active with landings estimates ranging from 400,000 to 700,000 red abalone taken per year (CDFG unpubl data). This northern fishery is unique in several ways (1) the use of SCUBA and commercial fishing has been prohibited for more than 40 years; (2) there are few access points along the rugged coastline; and (3) there are long periods of rough ocean conditions. The fishery in the north with an estimated 40,000 fishers is also managed using a combination of a minimum size limit (178 mm), season closures (December to March and July), daily (3/day) and yearly bag limits (24/year), and closed areas. The fishery generates a yearly average of $13.2 million (1985-1989) in tourism revenue to northern coastal communities (CDFG 1997). As with many fisheries, the recreational abalone fishery highly prizes large trophy animals.

Previous abalone reproduction studies have identified a number of important features associated with wild red abalone in northern California. First, there may be a difference in the size at maturity between males and females (Giorgi & DeMartini 1977); second, red abalone are capable of being highly fecund producing more than 12 million eggs (Giorgi & DeMartini 1977); and third, necrotic eggs have been found in mature females (Young & DeMartini 1970, Giorgi & DeMartini 1977). Laboratory conditioning (feeding kelp) has been shown to reduce the size at first maturity and increase estimates of fecundity by 260%, relative to wild abalone (Ault 1985). These results suggest that reproduction studies based on laboratory (or aquaculture) reared red abalone may not be relevant for wild populations. It is unknown whether the largest females have more necrotic eggs suggesting the possibility of reproductive senescence, as has been found in some other mollusks (Downing et al. 1993).

Red abalone fecundity and size at maturity was investigated for wild animals in northern California. Red abalone populations are centered along 480 km of rocky coastline dominated by bull kelp, Nereocystis, in Sonoma and Mendocino Counties north of San Francisco. Samples of red abalone were collected from Van Damme State Park (n = 393) over 4 years (2000-2003) and from the Point Arena Abalone Derby (n = 32) in 2000 and 2001. For each abalone, gonad volume was determined and samples of gonad tissue were prepared for histologic assessment of reproductive condition. Histologic evaluations were conducted to determine the maturation stage of the ovaries and testes. The potential fecundity of each female was estimated by multiplying estimated gonad volume by the mean number of mature eggs enumerated per slide excluding the necrotic eggs. We discuss the relationship between abalone size, and fecundity as well as reproductive senescence and it's implications for the ecology and management of red abalone populations in northern California.