FUNDAMENTALS OF CBA PRACTICE

Several aspects of seed fisheries for fish and invertebrate species can be considered of theoretical importance because they are fundamental to sustainable resource use, yet are either very little understood or extremely difficult to quantify. Therefore, the precautionary approach can, combined with the best available information and scientific reference, address key issues. Central to these, in relation to CBA, is the nature and extent of the linkage between wild seed collection of early life history stages and the condition of adult stocks, and how to manage different fishing sectors exploiting the same species.

The key biological information needed includes densitydependent effects among seed, and age-specific natural mortalities, both of which are virtually unknown for marine species with planktonic larval stages. These parameters are of critical important because they dictate how many seeds can be removed from the wild without affecting adult stock numbers.
For some species there is a known quantitative relationship between seed numbers and adults, while for most a relationship is not clear, the implications being that densitydependent effects and total mortality (i.e. natural plus fishing) levels probably have very different significance for different species, or depending on the age of the seed at capture.

Illustrative examples are lobster and shrimp. For tiger shrimp (Penaeus monodon), the post-larval fishery removes an estimated 90 percent of the seed population, which scientists believe is largely responsible for the heavy declines in the adult shrimp fishery off Bangladesh; in the case of Metapenaeus monoceros taken in the same general area, however, the adult fishery is in much better shape because, it is thought, there is little vulnerability of the post-larvae to fishing; less than 10 percent of this early stage is removed by fishing (SAPB, 2002). In the case of lobster, natural mortality estimates of Panulirus cygnus in western Australia suggest that this is regulated by density and is so high that even very large removals of pueruli are expected to have negligible effects on wild fisheries (Phillips et al., 2003). For the rock lobster (Jasus edwardsii), on the other hand, collection of pueruli potentially affect adult numbers and so there is interest in attaining “biological neutrality” whereby excessive seed removal does not compromise the adult fishery (Booth, Davis and Zane, 1999; Phillips et al., 2003; Gardner et al., 2006). The degree to which density-dependence is important will depend, in part, on whether stocks are habitat or recruitment limited. As fishing pressures increase, the latter becomes more likely and the relevance of density-dependence is likely to decline substantially. Very little is known about either natural mortality levels early in life or the extent to which density-dependent effects can and do occur under different conditions suggesting that a precautionary approach is applicable.
What is known about density-dependent effects and early natural mortality rates in finfishes subjected to seed fisheries that can assist management decisions? For a few reef fishes, although early juvenile survivorship varies among species in the first few weeks or months following settlement, indications are that natural mortality, very high at settlement, drops quickly during the first few months post-settlement (Sale and Ferrell, 1988; Koenig and Colin, 1999; Doherty et al., 2004). For such species, the intense exploitation of older juveniles (which have entered a low natural mortality stage) for CBA clearly increases fishing mortality directly on the stock as a whole, and the fishery should be managed to ensure that sufficient young are allowed to survive to reproduce for population persistence (Sadovy and Pet, 1998; Sadovy, 2000). A specific example of this is the fishery for grouper juveniles, many of which are several years old at capture and are removed prior to sexual maturation for grow-out in captivity to marketable size (Sadovy, 2000). This practice has increased over the last decade as adult stocks have become over-fished, the demand for seed has increased and fishers take ownership of caught fish as soon as they can rather than return undersize fish to the water (Sadovy, 2000; Sadovy et al., 2003). The impact of removing grouper juveniles at one week of age or less, compared to removing the same number of juveniles at 6 months or more (both practices are common) could mean the difference between healthy and devastated stocks. In the case of the older juveniles, their capture and the addition of feed is no different from the fattening of juveniles of bluefin tuna species.
A precautionary approach to seed fishery extraction rates, based on what is known currently, and acknowledging how much is still unknown about early life history stage dynamics, is to assume some degree of linkage between adult and seed fisheries and manage accordingly. This approach is already practised in the case of southern bluefin tuna through catch quotas on juveniles taken for fattening and for rock lobster (Gardner et al., 2006) (Fishery Status Reports, 2005; 2006); such quotas should be in numbers, rather than weight. Follow-up monitoring allows for adaptive management and adjustment of fishing levels as needed. This precautionary and adaptive approach is particularly relevant as seed fisheries intensify, with possibly billions rather than millions of seeds taken each year, and given how challenging it will be to collect the necessary natural mortality and density-dependent information. Key to such thinking is to acknowledge that the production of large numbers of eggs and larvae in pelagic spawning species is not a redundancy but a fundamental life history strategy. Enormous numbers are produced for a simple but compelling reason − the very low chances that any one propagule will survive to adulthood. Removing a significant number by fishing will inevitably further reduce that possibility to some extent, with negative implications inevitable beyond some threshold of removals. Fisheries for aquarium organisms and food fish, based on the capture of post-larval settlement phase fish, have been proposed as a viable alternative activity to adult capture, although nothing is known of the volumes of post-larvae that could be removed sustainably (Hair et al., 2004). A better understanding of such thresholds is needed and/or a means found to ensure it is not exceeded, thereby compromising affected fisheries and the livelihoods that depend on them.