In many marine invertebrates, the relationship between egg production and/or number of competent larvae and subsequent recruitment is often unknown. Estimating such relationship is critical for informing the management and conservation of exploited species because it determines a natural population's response to exploitation. In the current study, recruitment rates of abalone Haliotis fulgens and Haliotis corrugata were recorded by means of postlarval collectors inside and outside 2 marine reserves in 2 subsequent years. At the same time and locations, theoretical reproductive outputs were computed by combining estimates of abalone densities and size structure with literature data on size-dependent fecundity and size at sexual maturity. We found that observed settlement rates correlated positively and significantly with estimated reproductive output, suggesting that greater production of eggs by large and fecund individuals can result in predictably high recruitment rates. In addition, the slope of the linear relationship suggests that the proportion of eggs that settle and metamorphose to become postlarvae is between 0.2% and 0.3%. Although affected by great uncertainty, this value constitutes a unique estimate of larval survival for the genus Haliotis in a natural environment.
Establishing a stock—recruitment relationship (i.e., the relationship between parental stock size and subsequent recruitment) is a central problem of fish population dynamics, because stock-recruitment relationships regulate natural population size and determines its response to exploitation. Common models in population dynamics relating the recruitment R to spawner abundance N are the Beverton-Holt [R = aN/(1 + bN)] (Beverton & Holt 1957) and the Ricker [R = aN exp(—cN)] models (Ricker 1954). The same functional forms can be used to relate the successful settlement to the number of eggs or of competent larvae. In these models, parameter a has dimensions of recruitment per spawner, or of settler per eggs, or settler per larvae (in the latter cases, a can be viewed as an index of survival), whereas parameters b and c are the density-dependent terms. Parameter a gives the slope of the functions at the origin and, regardless of which model is chosen, is a crucial variable in fishery population models; it determines the maximum reproductive rate, the vulnerability to recruitment overfishing, and the extinction probability, as well as the resilience, recovery time, and likely response to protection (Mace 1994, Myers & Mertz 1998, Botsford et al. 1999, Barrowman & Myers 2000).
Recruitment is recognized as one of the most variable features of the life cycle of many marine fishes and invertebrates, including the marine gastropod abalone (genus Haliotis) (McShane 1995, Rogers-Bennett et al. 2004, Miner et al. 2006). Within this genus, the existence of a stock—recruitment relationship linking new recruits to reproductive adult densities is a matter of continued debate (Shepherd 1990, Rogers-Bennett et al. 2004). Although the longest time series (17 y) of stock and recruitment data for abalone found no clear relationship between adult populations and 2-y-old juveniles (Shepherd 1990), several studies show that greater adult densities support higher recruit abundance compared with depleted areas. An adult removal experiment by Prince et al. (1987) found a positive correlation between local adult density of Haliotis rubra and the number of newly settled individuals, suggesting that the relationship between recruits and reproductive individuals occurred at a local scale. Such localized effect in recruitment patterns is attributed to low larval dispersal distances documented in abalone, in the order of tens to hundred meters (Prince et al. 1987, McShane et al. 1988, Prince et al. 1988, Shepherd et al. 1992, Guzmán del Próo et al. 2000, Shanks et al. 2003, Micheli et al. 2012). Experimental studies by Rogers-Bennett et al. (2004) provided evidence that abundances of recruits for several Californian abalone species were significantly greater in Northern California, where abalone populations were found at relatively high densities, compared with sites in Southern California, where local populations were depleted. Recruitment failure was also found to be associated with low adult densities for black abalone Haliotis cracherodii, whereas higher adult population densities supported successful recruitment levels (Miner et al. 2006). Nevertheless, currently, no stock-recruitment relationship for the genus Haliotis has been proposed. Producing such a relationship is critical for demographic models for fishery management and for informing conservation strategies, such as the design of marine protected area networks and the possible translocation and aggregation of adults within depleted areas.
The goal of this study was to investigate the relationship between recruitment and reproductive potential for the green and pink abalone, Haliotis fulgens and Haliotis corrugata, using data from field surveys and recruitment experiments conducted in Isla Natividad, Baja California Sur, Mexico. In the fishing grounds surrounding the island, recruitment rates were quantified using larval collectors deployed on the seafloor inside and outside of 2 marine reserves created in 2006. In the same years and locations, the abundance and size structure of pink and green abalone were monitored yearly via scuba surveys. Last, detailed information on size-dependent fertility and size at sexual maturity for the 2 species was compiled from