Variation in length and age at sexual maturity of Atlantic groundfish: a reply

Synopsis The assertion has been made by Halliday (1987) that trends in size and age at maturity of Atlantic groundfish published by Beacham (1983a, b, c, d, e, f) are artifacts induced by errors in determining the sex of an individual, distinguishing between immature and mature fish, sampling fish outside of the regular spawning season, and by nonrandom sampling of the population. In particular, Halliday asserts that for the Atlantic argentine,Argentina silus analysis, the conclusions of Beacham (1983a) that: (1) median length at sexual maturity declined over time; and (2) males matured at older ages than did females are invalid owing to biases in both sampling and analysis. In fact, if some of the biases indicated by Halliday were significant, then the decline in median length at sexual maturity is enhanced and the conclusions of Beacham (1983a) reinforced. Size and age at sexual maturity are dynamic characters in many vertebrate populations, and the fact that they should change for Atlantic groundfish should not be surprising given variable exploitation patterns in the fisheries since 1960.     

Diel food utilization by woundfin,Plagopterus argentissimus,in Virgin River,Arizona

Synopsis Woundfin, Plagopterus argentissimus, fed predominantly on simuliid larvae during the day, and shifted to the larger Hydropsyche spp. at night. Ephemeropteran larvae were eaten nearly uniformly throughout the 24 hour period. Mean weight of food consumed varied from 20–40 mg per individual (0.7–2.5% body weight) throughout the 24 h period. Our estimated daily ration of about 8% body weight, at temperatures that varied between 15–25°C, is based upon our measure of mean gut contents over a 24h period, combined with intestinal evacuation rates suggested for other cyprinids in the literature. The relatively continuous feeding pattern suggests a foraging strategy that would minimize competition with other species for food during June, a period of minimum annual food abundance in Virgin River.     

To produce many small or few large eggs: a size-independent reproductive tactic of fish

Summary We demonstrate here the existence of a range of size-independent reproductive tactics in teleostean fish involving the allocation of a size-dependent reproductive effort between fecundity and egg size. Despite considerable evidence that larger eggs and the larvae hatching from them are more likely to survive than smaller ones, we found no evidence of evolutionary trends towards greater egg sizes. Fish with pelagic eggs tend to spawn many, and therefore small, eggs, whereas demersal spawners tend to produce large, and therefore few, eggs. Maximizing egg number should increase the number of eggs hatching in suitable locations in the patchy pelagic environment and, hence, increase the reproductive success of pelagic spawners. On the other hand, the reproductive success of demersal spawners, which reduce the variance in growing conditions experienced by the off spring, should be more dependent on the survival of the individual larvae, which increases as egg size increases.     

Experimental evaluation of a minnow trap for small lotic fish

A minnow trap that operates in various flow regimes in streams and allows sampling of small fish from stream bed microhabitats was developed. In laboratory and field tests, the most efficient trap design for capturing and retaining various species of fish had one funnel oriented downstream, a plexiglass body, and commercial trout food as bait. These lightweight traps can be set in a wide range of current velocities and depths, and can be useful in investigations that examine the microhabitat use, diel activity patterns or population densities of small lotic fish. Guidelines for the trap's use and for quick verification of capture success in new situations are suggested.     

Assessing the impact of episodic pollution

The increased tightening of controls on industrial and municipal wastewater discharges has resulted in steady improvements in the quality of many important rivers over recent years. However, episodic pollution, particularly from farm wastes and combined sewer overflows continues to pose a major problem, and is one of the main causes of poor quality rivers today. Despite our acknowledgement of this continuing problem, very little is known of the mechanistic basis of responses and recovery of aquatic organisms and communities exposed to intermittent pulses of common pollutants. The majority of ecotoxicological studies to date have been concerned with the effects of continuous exposure. Although such studies may provide a means of predicting the impact of episodic pollution events, a more appropriate test design would be to assess toxicity under pulsed and fluctuating exposure. Studies should also include a post-exposure observation period and should consider recovery of individuals and communities. This paper reviews the results of reported studies relevant to the investigation of episodic pollution and pays particular attention to the effects of magnitude, duration and frequency of exposure. Results of field investigations using an in situ bioassay technique are also presented to emphasize the importance of field validation of proposed water quality criteria for intermittent pollution events.     

Social organization and spawning in the Atlantic sharpnose puffer,Canthigaster rostrata (Tetraodontidae)

Synopsis Social organization and spawning in the sharpnose pufferCanthigaster rostrataere studied on a reef in the San Blas Islands, Panama. Sexes were dimorphic. In mixed coral and rubble habitat, females defended territories against other females and small males. From one to six female territories were included within the territories of certain large males. These haremic males visited their females and patrolled their territories throughout the day. Smaller, non-haremic males occupied territories or home ranges within or adjacent to those of haremic males or were wanderers. Spawning between a haremic male and a territorial female occurred within the female's territory. The female prepared an algal nest into which demersal eggs were deposited. There was no parental care. Eggs were spherical, translucent, and measured approximately 0.66 mm in diameter. Larvae were about 1.4 mm TL and closely resembled those of other species ofCanthigaster.     

Underwater observations of piranhas in western Brazil

Synopsis The behaviour of three piranha species,Serrasalmus marginatus, S. spilopleura, andPygocentrus nattereri, and their prey fishes was studied underwater in the Pantanal region, Mato Grosso, Brazil. General habits, predatory tactics, feeding behaviour, and social interactions while foraging, as well as defensive tactics of prey fishes were observed.S. marginatus is solitary whereas the other two species live in shoals; their agonistic behaviour varies accordingly, the simplest being displayed by the solitary species. Predatory tactics and feeding behaviour also vary:S. spilopleura shows the most varied diet and highly opportunistic feeding strategy, which includes aggressive mimicry. The solitaryS. marginatus, besides fin and scale-eating, occasionally cleans larger individuals ofP. nattereri. Several cichlid species display defensive tactics clearly related to piranha attacks: tail protecting, watching, and confronting the predator are the most commonly observed behaviours. Piranhas seem to strongly influence use of habitat, social structure, and foraging mode of the fish communities.     

Petersen and removal population size estimates: combining methods to adjust and interpret results when assumptions are violated

Synopsis We present ways to test the assumptions of the Petersen and removal methods of population size estimation and ways to adjust the estimates if violations of the assumptions are found. We were motivated by the facts that (1) results of using both methods are commonly reported without any reference to the testing of assumptions, (2) violations of the assumptions are more likely to occur than not to occur in natural populations, and (3) the estimates can be grossly in error if assumptions are violated. We recognize that in many cases two days in the field is the most time fish biologists can spend in obtaining a population estimate, so the use of alternative models of population estimation that require fewer assumptions is precluded. Hence, for biologists operating with these constraints and only these biologists, we describe and recommend a two-day technique that combines aspects of both capture-recapture and removal methods. We indicate how to test: most of the assumptions of both methods and how to adjust the population estimates obtained if violations of the assumptions occur. We also illustrate the use of this combined method with data from a field study. The results of this application further emphasize the importance of testing the assumptions of whatever method is used and making appropriate adjustments to the population size estimates for any violations identified.     

Effect of defolliculation and 17α-hydroxy, 20β-dihydroprogesterone on cyclic AMP level in full-grown oocytes of the rainbow trout,Salmo gairdneri

The changes in cAMP were followed in trout oocytes incubated in vitro after defolliculation performed by either enzymatic or manual dissection. Both defolliculation methods induced a highly significant rise in oocyte cAMP level (4.5 times the basal level of control [follicle-enclosed oocytes], after 6 h). Treatment of defolliculated oocytes with 17α-hydroxy,20β-dihydroprogesterone (17α,20β-OH-P) (10^?6 M), which induced oocyte maturation (germinal vesicle breakdown [GVBD]) was able, first, to interrupt the increase of oocyte cAMP level promoted by defolliculation and then to lower this level significantly down to values that still remained higher than folliculated controls. Very low concentrations of 17α,20β-OH-P (1.38–55.6 10^?9 M), or physiological doses of testosterone (0.35 10^?6 M, in the range found in vivo before ovulation) were able to induce a similar decrease of oocyte cAMP level without inducing GVBD. Under the same experimental conditions estradiol (0.35 10^?6 M) exhibited no action. These results suggest that some factor(s) originating in the follicle (FIF), inhibit the oocytes' tendency to accumulate cAMP before the final surge of 17α,20β-OH-P. This factor might be a follicular steroid such as testosterone or nonmaturing concentrations of 17α,20β-OH-P. Moreover our data favour the hypothesis that the final surge of 17α,20β-OH-P could induce distinct intraoocyte mechanisms: the first induces an irreversible blockage of cAMP level before the inhibitory action of the FIF is suppressed by ovulation, and the second mechanism leads to GVBD.     

Polyamines in liver and their influence on chromatin condensation after 17-β estradiol treatment of Atlantic salmon

Atlantic salmon (Salmo salar) were treated with 17- estradiol to induce vitellogenin synthesis in liver. This led to an increase in liver wet weight and total DNA. After incubation with micrococcal nuclease (EC 3.1.31.1) less soluble chromatin was obtained from nuclei of the estradiol treated than the control fish, but active gene regions were solubilized by the nuclease. Thus, in the estradiol treated fish soluble mononucleosomes contained hybridizable vitellogenin gene sequences. As a result of estradiol treatment the content in total liver of putrescine rose 3-fold, that of spermidine 2-fold, while spermine was unchanged. In muscle no significant changes were observed. The regulatory functions of polyamines during gene expression were investigated by binding (¹⁴C)spermine to isolated liver nuclei depleted of endogenous polyamines. The number of binding sites was higher in nuclei of estradiol treated than control fish. (^14C)spermine associated preferentially with micrococcal nuclease insensitive chromatin. Thus, the high content of putrescine and spermidine in liver supported the view of polyamine accumulation in proliferating tissues. The preferential binding to condensed chromatin indicated a stabilizing effect of polyamines on the organization of inactive chromatin structures.Abbreviations MNase micrococcal nuclease - PMSF phenylmethylsulfonylfluoride