Are aggregation-fisheries sustainable? Reef fish fisheries as a case study

Reef fish spawning aggregations are attractive and often lucrative to fish but particularly vulnerable to fishing, with many cases of declines or extirpations. While awareness of the risks of aggregation exploitation has grown substantially in the tropical western Atlantic in the last decade, the phenomenon of aggregation-spawning is little known in the vast Indo-Pacific region where few aggregations are managed or monitored, and are rarely considered in marine-protected area designations. Even in the tropical western Atlantic and Caribbean, marine-protected area planning, until recently, did not typically consider spawning aggregations. Available data and analyses of aggregation-fisheries and aggregating species strongly suggest that: (1) the majority of known aggregations that are exploited are yielding declining landings; (2) aggregating species show greatest overall declines in local fisheries when their aggregations are also exploited; (3) from an economic perspective, aggregation fishing may yield lower prices for fish, or aggregations may be more valuable unexploited, as a source of fish for local fisheries or as tourist attractions; (4) hyperstability can mask declines in aggregation-fisheries, based on fishery-dependent data; (5) monitoring of aggregation catches by either fishery-dependent or fishery-independent means is deceptively challenging. There are also possible ecosystem-level consequences of reducing or eliminating spawning aggregations. We conclude that aggregation-fisheries are likely to be sustainable only for limited subsistence-level use, that the precautionary principle should be more widely applied in aggregation-fisheries and that, despite growing interest in aggregations over the last decade, few of the key biological questions necessary for effective management have been, or are being, addressed.     

Stability in the historical pattern of genetic structure of Newfoundland cod (Gadus morhua) despite the catastrophic decline in population size from 1964 to 1994

We report on evidence of long term stability inthe geographic pattern of geneticdifferentiation among cod (Gadus morhua)collected from 5 spawning banks offNewfoundland and Labrador over a periodspanning three decades (1964–1994) and 2orders of magnitude of population sizevariation. Six microsatellite DNA lociamplified from archived otoliths (1964 and1978) and contemporary (1990s) tissue samplesrevealed fidelity to natal spawning banks overthis period. A two level (spawning bank anddecade) hierarchical and multilocus AMOVAindicated that 1.55% of the total variation inallele frequencies could be attributed(P = 0.036) to spatial structure while novariance component could be attributed totemporal changes. A finer scale analysis amongcod from just 3 of these spawning banksreveals, however, evidence consistent with somepost-collapse mixing between cod from twobanks. In the context of fisheries managementand conservation, the survival of the spatialpattern of genetic differentiation during thepopulation collapse suggests that if recoveryeventually occurs it will likely be throughpopulation re-growth in situ rather thanby migratory influx.     

The reproductive and feeding biology of Schilbe intermedius Rüppell in the Okavango Delta,Botswana

The reproductive and feeding biology of 314 Schilbe intermedius Rüppell 1832 collected in the Okavango Delta, Botswana, between April 1985 and February 1986 during low, intermediate and high water level were investigated. S. intermedius matures at 130 mm standard length and is a monocyclic spawner. Females predominate in the population and have an average fecundity of 27107 eggs. In the northern riverine floodplain of the Delta fish spawn in mid-summer during the peak flood. In the southern drainage rivers fish spawn in spring, which is two to three months after the arrival of the winter flood waters. S. intermedius is an opportunistic predator with a diet consisting largely of fish 41%, aquatic larvae 25%, terrestrial insects 14%, aquatic insects 7% and crustacea 5%. Juvenile S. intermedius feed predominantly on insect larvae, while adults are largely piscivorous.     

Sneaking by harem masters of the sandperch, Parapercis snyderi

Reproductive behavior of the sandperch Parapercis snyderi was studied in the coastal waters of Shikoku Island, Japan. This fish is a monandric and protogynous hermaphrodite. Males maintained home ranges of ca. 10 m² that slightly overlapped with each other. Harems included on average 2.45 females that were spaced out from each other. Males mated successively in pairs with the harem members within two hours before sunset. To release gametes, the pair rushed upward 10–200 cm. The pair spawning was sometimes parasitized by sneaking of an adjacent harem master, which had usually finished mating with his own mates. More than 80% of the sneaking rushes reached the apex of the pair rise, suggesting a high success rate of sneaking. Such sneaking by harem masters has not been reported in the genus Parapercis. The comparative reproductive ecology of this genus suggested that sneaking in P. snyderi is due to the close proximity of adjacent harems.     

Reconstructing migratory patterns of fish based on environmental influences on otolith chemistry

The analysis of elements in calcifiedstructures of fish (e.g., otoliths) todiscriminate among fish stocks and determineconnectivity between populations is becomingwidespread in fisheries research. Recently, theconcentrations of elements in otoliths arebeing analysed on finer scales that allow thedetermination of a continuous record of otolithchemistry over a fish's entire life history.These elemental concentrations can potentiallybe used to reconstruct migration patterns,based upon the influence that water chemistry,temperature, and salinity have on otolithchemistry. In doing so, assumptions are madeabout how environmental and biological factorsinfluence the concentration of elements in fishotoliths. However, there have been fewexperiments that have tested crucialassumptions regarding what influences elementaluptake and incorporation into fish otoliths.Specifically, knowledge regarding interactionsamong environmental variables, such as theambient concentration of elements in water,temperature, and salinity, and how they mayaffect otolith chemistry, is limited.Similarly, our understanding of the rate atwhich elements are incorporated into otolithsand the implications this may have forinterpretations is lacking. This reviewdiscusses methods of determining movement offish, the development of otolith research, andsome physiological aspects of otoliths (e.g.,pathways of elemental uptake). The types ofanalysis techniques that will lead to reliableand accurate migratory reconstructions areoutlined. The effects that have on otolith chemistry arereviewed with the specific aim of highlightingareas lacking environmentalvariables in experimental data. Theinfluences of the rate of elementalincorporation and ontogeny on otolith chemistryare also addressed. Finally, future researchdirections are suggested that will fill thegaps in our current knowledge of otolithchemistry. Hypotheses that need to be tested inorder to reconstruct the migratory histories offish are outlined, in a bid to clarify thedirection that research should take beforecomplex reconstructions are attempted.     

Reproductive segregation among the Barbus intermedius complex of Lake Tana,Ethiopia. An example of intralacustrine speciation?

Reproductive segregation among the large barbs Barbus intermedius complex, Cyprinidae , of Lake Tana, Ethiopia, was investigated in the mouths and upstream reaches of rivers tributary to the lake, during the spawning seasons of 1993 and 1994. The percentage of running fish of four Barbus morphotypes [Acute (Ac), Bigmouth big-eye (Be), Intermedius (In), and Shorthead (Sh)] peaks with the highest water level of the lake. Large differences are apparent in the minimum fork length of running fish among morphotypes [c. 12 cm for male Sh and 28 cm for male Bigmouth small-eye (Se)]. Significant differences occur in temporal, but especially in spatial patterns of the relative abundance of running morphotypes, when the lake is compared with the four major river mouths and their upstream reaches. These different relative abundances are clearest among Ac, Be, In, Sh and Troutlike (Tr). Also, the absence from the rivers of some morphotypes that are usually abundant in the lake itself [Barbel (Ba), Dark (Da), Lip (Li), and Zurki (Zu)], and the abundant presence in the rivers of Tr, rare in the lake, strengthen the hypothesis that spatial and/or temporal spawning segregation is present among the morphotypes. There are indications that Bighead (Bh) and Zu spawn in the lake itself, while other morphotypes were only found running in or near rivers. Combined with the morphological, ecological, and genetic differences, these data suggest the existence of several Barbus species, forming a unique species flock, which probably evolved within the lake. The presence of several, instead of one Barbus species complicates management of sustainable fisheries and protection of the lake's biodiversity. Present results show the importance of the rivers for Barbus reproduction and stress the urgent need for careful management of fishing activities in spawning times and areas.