Modeling seabird group size: implications for ecological impact assessments

The purpose of this project is to model seabird flock size data to provide recommendations to the Bureau of Ocean and Energy Management for offshore wind turbine placement. Our hypothesis is that ecological characteristics influence which statistical distribution will provide the best fit to seabird flock size data. To test this, seabird species can be grouped based on shared ecological traits, such as foraging mechanism or diet.

Use of skeletochronological analysis to estimate the age of leatherback sea turtles Dermochelys coriacea in the western North Atlantic

Although growth rate and age data are essential for leatherback management, estimates of these demographic parameters remain speculative due to the cryptic life history of this endangered species. Skeletochronological analysis of scleral ossicles obtained from 8 captive, known-age and 33 wild leatherbacks originating from the western North Atlantic was conducted to characterize the ossicles and the growth marks within them. Ages were accurately estimated for the known-age turtles, and their growth mark attributes were used to calibrate growth mark counts for the ossicles from wild specimens. Due to growth mark compaction and resorption, the number of marks visible at ossicle section tips was consistently and significantly greater than the number visible along the lateral edges, demonstrating that growth mark counts should be performed at the tips so that age is not underestimated. A correction factor protocol that incorporated the trajectory of early growth increments was used to estimate the number of missing marks in those ossicles exhibiting resorption, which was then added to the number of observed marks to obtain an age estimate for each turtle. A generalized smoothing spline model, von Bertalanffy growth curve, and size-at-age function were used to obtain estimates of age at maturity for leatherbacks in the western North Atlantic. Results of these analyses suggest that median age at maturation for leatherbacks in this part of the world may range from 24.5 to 29 yr. These age estimates are much greater than those proposed in previous studies and have significant implications for population management and recovery.

The effects of size-selective fisheries on the stock dynamics of and sperm limitation in sex-changing fish

Fisheries models have traditionally focused on patterns of growth, fecundity, and survival of fish. However, reproductive rates are the outcome of a variety of interconnected factors such as life-history strategies, mating patterns, population sex ratio, social interactions, and individual fecundity and fertility. Behaviorally appropriate models are necessary to understand stock dynamics and predict the success of management strategies. Protogynous sex-changing fish present a challenge for management because size-selective fisheries can drastically reduce reproductive rates. We present a general framework using an individual-based simulation model to determine the effect of life-history pattern, sperm production, mating system, and management strategy on stock dynamics. We apply this general approach to the specific question of how size-selective fisheries that remove mainly males will impact the stock dynamics of a protogynous population with fixed sex change compared to an otherwise identical dioecious population. In this dioecious population, we kept all aspects of the stock constant except for the pattern of sex determination (i.e. whether the species changes sex or is dioecious). Protogynous stocks with fixed sex change are predicted to be very sensitive to the size-selective fishing pattern. If all male size classes are fished, protogynous populations are predicted to crash even at relatively low fishing mortality. When some male size classes escape fishing, we predict that the mean population size of sex-changing stocks will decrease proportionally less than the mean population size of dioecious species experiencing the same fishing mortality. For protogynous species, spawning-per-recruit measures that ignore fertilization rates are not good indicators of the impact of fishing on the population. Decreased mating aggregation size is predicted to lead to an increased effect of sperm limitation at constant fishing mortality and effort. Marine protected areas have the potential to mitigate some effects of fishing on sperm limitation in sex-changing populations.

Age determination and growth of the night shark (Carcharhinus signatus) off the northeastern Brazilian coast

Age and growth of the night shark (Carcharhinus signatus) from areas off northeastern Brazil were determined from 317 unstained vertebral sections of 182 males (113–215 cm total length [TL]), 132 females (111.5–234.9 cm) and three individuals of unknown sex (169–242 cm). Although marginal increment (MI) analysis suggests that band formation occurs in the third and fourth trimesters in juveniles, it was inconclusive for adults. Thus, it was assumed that one band is formed annually. Births that occur over a protracted period may be the most important source of bias in MI analysis. An estimated average percent error of 2.4% was found in readings for individuals between two and seventeen years. The von Bertalanffy growth function (VBGF) showed no significant differences between sexes, and the model derived from back-calculated mean length at age best represented growth for the species (L∞=270 cm, K=0.11/yr, t0=–2.71 yr) when compared to the observed mean lengths at age and the Fabens’ method. Length-frequency analysis on 1055 specimens (93–260 cm) was used to verify age determination. Back-calculated size at birth was 66.8 cm and maturity was reached at 180–190 cm (age 8) for males and 200–205 cm (age ten) for females. Age composition, estimated from an age-length key, indicated that juveniles predominate in commercial catches, representing 74.3% of the catch. A growth rate of 25.4 cm/yr was estimated from birth to the first band (i.e. juveniles grow 38% of their birth length during the first year), and a growth rate of 8.55 cm/yr was estimated for eight- to ten-year-old adults.

Latitudinal and seasonal egg-size variation of the anchoveta (Engraulis ringens) off the Chilean coast

The anchoveta Engraulis ringens is widely distributed along the eastern South Pacific (from 4° to 42°S; Serra et al., 1979) and it has also supported one of the largest fisheries of the world over the last four decades. However, there are few interpopulation comparisons for either the adult or the younger stages. Reproductive traits, such as fecundity or spawning season length, are known to vary with latitude for some fish species (Blaxter and Hunter, 1982; Conover, 1990; Fleming and Gross, 1990; Castro and Cowen, 1991), and latitudinal trends for some early life history traits, such as egg size and larval growth rates, have been reported for others clupeiforms and other fishes (Blaxter and Hempel, 1963; Ciechomski, 1973; Imai and Tanaka, 1987, Conover 1990, Houde 1989). However, there is no published information on potential latitudinal trends during the adult or the early life history of the anchoveta, even though this type of information may help in understanding recruitment variability, especially during recurring large scale events (such as El Niño or La Niña) that affect the entire species range.

Diet shifts of juvenile red snapper (Lutjanus campechanus) with changes in habitat and fish size

We examined the diets and habitat shift of juvenile red snapper (Lutjanus campechanus) in the northeast Gulf of Mexico. Fish were collected from open sand-mud habitat (little to no relief), and artificial reef habitat (1-m3 concrete or PVC blocks), from June 1993 through December 1994. In 1994, fish settled over open habitat from June to September, as shown by trawl collections, then began shifting to reef habitat — a shift that was almost completed by December as observed by SCUBA visual surveys. Stomachs were examined from 1639 red snapper that ranged in size from 18.0 to 280.0 mm SL. Of these, 850 fish had empty stomachs, and 346 fish from open habitat and 443 fish from reef habitat contained prey. Prey were identified to the lowest possible taxon and quantified by volumetric measurement. Specific volume of particular prey taxa were calculated by dividing prey volume by individual fish weight. Red snapper shifted diets with increasing size. Small red snapper (<60 mm SL) fed mostly on chaetognaths, copepods, shrimp, and squid. Large red snapper (60–280 mm SL) shifted feeding to fish prey, greater amounts of squid and crabs, and continued feeding on shrimp. We compared red snapper diets for overlapping size classes (70–160 mm SL) of fish that were collected from both habitats (Bray-Curtis dissimilarity index and multidimensional scaling analysis). Red snapper diets separated by habitat type rather than fish size for the size ranges that overlapped habitats. These diet shifts were attributed to feeding more on reef prey than on open-water prey. Thus, the shift in habitat shown by juvenile red snapper was reflected in their diet and suggested differential habitat values based not just on predation refuge but food resources as well.

Maturity, ovarian cycle, fecundity, and age-specific parturition of black rockfish (Sebastes melanops)

From 1995 to 1998, we collected female black rockfish (Sebastes melanops) off Oregon in order to describe their basic reproductive life history and determine age-specific fecundity and temporal patterns in parturition. Female black rockfish had a 50% probability of being mature at 394 mm fork length and 7.5 years-of-age. The proportion of mature fish age 10 or older significantly decreased each year of this study, from 0.511 in 1996 to 0.145 in 1998. Parturition occurred between mid-January and mid-March, and peaked in February. We observed a trend of older females extruding larvae earlier in the spawning season and of younger fish primarily responsible for larval production during the later part of the season. There were differences in absolute fecundity at age between female black rockfish with prefertilization oocytes and female black rockfish with fertilized eggs; fertilized-egg fecundity estimates were considered superior. The likelihood of yolked oocytes reaching the developing embryo stage increased with maternal age. Absolute fecundity estimates (based on fertilized eggs) ranged from 299,302 embryos for a 6-year-old female to 948,152 embryos for a 16-year-old female. Relative fecundity (based on fertilized eggs) increased with age from 374 eggs/g for fish age 6 to 549 eggs/g for fish age 16.

Reproductive dynamics of female spotted seatrout (Cynoscion nebulosus) in South Carolina

We describe reproductive dynamics of female spotted seatrout (Cynoscion nebulosus) in South Carolina (SC). Batch fecundity (BF), spawning frequency (SF), relative fecundity (RF), and annual fecundity (AF) for age classes 1−3 were estimated during the spawning seasons of 1998, 1999, and 2000. Based on histological evidence, spawning of spotted seatrout in SC was determined to take place from late April through early September. Size at first maturity was 248 mm total length (TL); 50% and 100% maturity occurred at 268 mm and 301 mm TL, respectively. Batch fecundity estimates from counts of oocytes in final maturation varied significantly among year classes. One-year-old spotted seatrout spawned an average of 145,452 oocytes per batch, whereas fish aged 2 and 3 had a mean BF of 291,123 and 529,976 oocytes, respectively. We determined monthly SF from the inverse of the proportion of ovaries with postovulatory follicles (POF) less than 24 hours old among mature and developing females. Overall, spotted seatrout spawned every 4.4 days, an average of 28 times during the season. A chronology of POF atresia for water temperature >25°C is presented. Length, weight (ovary-free), and age explained 67%, 65%, and 58% of the variability in BF, respectively. Neither RF (number of oocytes/g ovary-free weight) nor oocyte diameter varied significantly with age. However, RF was significantly greater and oocyte diameter was smaller at the end of the spawning season. Annual fecundity estimates were approximately 3.2, 9.5, and 17.6 million oocytes for each age class, respectively. Spotted seatrout ages 1−3 contributed an average of 29%, 39%, and 21% to the overall reproductive effort according to the relative abundance of each age class. Ages 4 and 5 contributed 7% and 4%, respectively, according to predicted AF values.

A method to improve size estimates of walleye pollock (Theragra chalcogramma) Atka mackerel (Pleurogrammus monopterygius) consumed by pinnipeds: digestion correction factors applied to bones and otoliths recovered in scats

The lengths of otoliths and other skeletal structures recovered from the scats of pinnipeds, such as Steller sea lions (Eumetopias jubatus), correlate with body size and can be used to estimate the length of prey consumed. Unfortunately, otoliths are often found in too few scats or are too digested to usefully estimate prey size. Alternative diagnostic bones are frequently recovered, but few bone-size to prey-size correlations exist and bones are also reduced in size by various degrees owing to digestion. To prevent underestimates in prey sizes consumed techniques are required to account for the degree of digestion of alternative bones prior to estimating prey size. We developed a method (using defined criteria and photo-reference material) to assign the degree of digestion for key cranial structures of two prey species: walleye pollock (Theragra chalcogramma) and Atka mackerel (Pleurogrammus monopterygius). The method grades each structure into one of three condition categories; good, fair or poor. We also conducted feeding trials with captive Steller sea lions, feeding both fish species to determine the extent of erosion of each structure and to derive condition-specific digestion correction factors to reconstruct the original sizes of the structures consumed. In general, larger structures were relatively more digested than smaller ones. Mean size reduction varied between different types of structures (3.3−26.3%), but was not influenced by the size of the prey consumed. Results from the observations and experiments were combined to be able to reconstruct the size of prey consumed by sea lions and other pinnipeds. The proposed method has four steps: 1) measure the recovered structures and grade the extent of digestion by using defined criteria and photo-reference collection; 2) exclude structures graded in poor condition; 3) multiply measurements of structures in good and fair condition by their appropriate digestion correction factors to derive their original size; and 4) calculate the size of prey from allometric regressions relating corrected structure measurements to body lengths. This technique can be readily applied to piscivore dietary studies that use hard remains of fish.

Temporal changes in population density, fecundity, and egg size of the Hawaiian spiny lobster (Panulirus marginatus) at Necker Bank, Northwestern Hawaiian Islands

Fecundity (F, number of brooded eggs) and egg size were estimated for Hawaiian spiny lobster (Panulirus marginatus) at Necker Bank, North-western Hawaiian Islands (NWHI), in June 1999, and compared with previous (1978–81, 1991) estimates. Fecundity in 1999 was best described by the power equations F = 7.995 CL 2.4017, where CL is carapace length in mm (r2=0.900), and F = 5.174 TW 2.758, where TW is tail width in mm (r2=0.889) (both n=40; P< 0.001). Based on a log-linear model ANCOVA, size-specific fecundity in 1999 was 18% greater than in 1991, which in turn was 16% greater than during 1978–81. The additional increase in size-specific fecundity observed in 1999 is interpreted as evidence for further compensatory response to decreased lobster densities and increased per capita food resources that have resulted either from natural cyclic declines in productivity, high levels of harvest by the commercial lobster trap fishery, or both.