Reef Size Research Articles

Silurian reef-dwelling pentamerid brachiopods, Wisconsin and Illinois, USA

Eight species of pentamerid brachiopods, assigned toApopentamerus, Capelliniella, Kirkidium, Lissocoelina, Rhipidium, Supertrilobus andStenopentamerus, occur in stromatoporoid-coral reefs of the Silurian (Wenlockian) Racine Formation, Wisconsin and Illinois, USA. One new taxon,Rhipidium kuglitschi n.sp., is described. In contrast to level-bottom communities, where single pentamerid species occur in great numbers over large areas, pentamerids were generally rare members of the Racine reef fauna, and most species have localized distributions. An increase in diversity from one pentamerid species in the north to six species in the south correlates with a southward increase in reef size. Four morphologic groups of pentamerid species differed in the shape of the mantle cavity, which suggests different dynamics of suspension-feeding. Where two or more species are present in single reefs, co-occurrence of these groups represents a partitioning of the pentamerid niche. The relatively high diversity, local distributions, and morphologic distinctions among the Racine pentamerid fauna represent an evolutionary radiation and functional specialization of pentamerids in Wenlock reefs.

Artificial Reefs: The Importance of Comparisons with Natural Reefs

Methods used to evaluate the performance of an artificial reef will vary according to the purpose for which the reef was built. To determine how well artificial reefs mitigate losses due to human activities on natural reefs, the performance of artificial reefs should be evaluated using contemporaneous comparisons with relatively undisturbed natural reefs. Unfortunately, comparisons between artificial and natural reefs are typically confounded by differences in reef size, age, and isolation. We compared colonization and subsequent assemblage structure of reef fishes on coral and artificial (concrete block) reefs in which reef size, age, and isolation were standardized. Species richness and fish abundance (all species combined) were greater on reefs of natural rather than artificial structure, but substantial differences in species composition were not detected. Our results suggest that artificial reefs with structural complexity and other abiotic and biotic features similar to those of natural reefs will best mitigate in-kind losses of reef fish populations and assemblages from natural reefs. Because of the open nature of most reef fish populations, estimating the contribution of artificial reefs in attracting v producing reef fishes will require a regional assessment of rates of demographic processes on both artificial and nearby natural reefs.

The influence of habitat on the size distribution of groupers in the upper Florida Keys

The influence of habitat on the size distribution of groupers was examined at sites in the middle and upper Florida Keys. Transects were used to quantify the size distribution of groupers at study sites. There were significant differences in the size distribution of groupers within and among reef community types related to differences in species composition and patch reef size. Groupers with a ‘giant’ life-history style (Sullivan & de Garine 1994) were more abundant, but smaller, on inshore patch reefs than offshore reef community types. However, grouper species with a ‘dwarf’ life-history style (Sullivan & de Garine 1994) showed an opposite pattern with a lower abundance, but larger size, inshore than offshore. The length category of groupers constituting the majority of individuals observed on patch reefs was inversely related to reef size. Graysby Epinephelus cruentatus were shown to recruit to deeper (15–20 m), low-relief habitats offshore. Several factors influenced the size distribution of groupers in the study sites including habitat type, spear-fishing, competition, predation, and recruitment.

Use of Egg Traps to Investigate Lake Trout Spawning in the Great Lakes

Disk-shaped traps were used to examine egg deposition by lake trout (Salvelinus namaycush) at 29 sites in the Great Lakes. The main objectives were to; first, evaluate the disk trap as a device for sampling lake trout eggs in the Great Lakes, and second, summarize what has been learned about lake trout spawning through the use of disk traps. Of the 5,085 traps set, 60% were classified as functional when retrieved. Evidence of lake trout egg deposition was documented in each of the lakes studied at 14 of 29 sites. A total of 1,147 eggs were trapped. The percentage of traps functioning and catch per effort were compared among sites based on depth, timing of egg deposition, distance from shore, size of reef, and type of reef (artificial or natural). Most eggs were caught on small, shallow, protected reefs that were close to shore. Use of disk traps on large, shallow, unprotected offshore reefs or along unprotected shorelines was generally unsuccessful due to the effects of heavy wind and wave action. Making multiple lifts at short intervals, and retrieval before and re-deployment after storms are recommended for use in exposed areas. On large reefs, preliminary surveys to identify preferred lake trout spawning habitat may be required to deploy disk traps most effectively. Egg deposition by hatchery-reared fish was widespread throughout the Great Lakes, and the use of artificial structures by these fish was extensive.

Lake Trout Spawning Habitat in the Great Lakes — a Review of Current Knowledge

We review existing information on lake trout spawning habitat, which might indicate whether habitat is now a limiting factor in lake trout reproductive success. Lake trout spawning habitat quality is defined by the presence or absence of olfactory cues for homing, reef location with respect to the shoreline, water depth, proximity to nursery areas, reef size, contour, substrate size and shape, depth of interstitial spaces, water temperature at spawning time, water quality in interstitial spaces, and the presence of egg and fry predators. Data on factors which attracted native spawners to spawning reefs are lacking, due to the absence of historic data on egg deposition. No direct evidence of egg deposition has been collected from sites deeper than 18 m. Interstitial space and, therefore, substrate size and shape, appear to be critical for both site selection by adults and protection of eggs and fry. Water quality is clearly important for egg incubation, but the critical parameters which define water quality have not yet been well determined in the field. Exposure to wave energy, dictated in part by reef location, may maintain high water quality but may also damage or dislodge eggs. The importance of olfactory cues, water temperature, and proximity to nursery habitat to spawning trout is unclear. Limited data suggest that egg and fry predators, particularly exotic species, may critically affect fry production and survival. Although availability of physical spawning habitat is probably not limiting lake trout reproduction, changes in water quality and species composition may negatively affect early life stages. This review of habitat factors that affect early life stages of lake trout suggests several priorities for research and management.

Factors influencing the abundance of the seven-spined morphotype of Spirobranchus polycerus (Schmarda), (Serpulidae), on upright blades of the hydrozoan coral, Millepora complanata

This study, on the seven-spined morphotype of Spirobranchus polycerus, a serpulid polychaete commensal with the hydrozoan coral Millepora complanata Lamarck examines live worm abundance, net recruitment and the probability of mortality, on single blades of coral at four Barbados fringing reefs, Heron Bay (HB), Greensleeves (G), Sandridge (S) and Six Men's Bay (SMB). Variation in the number of worms blade-1 over the part 5 to 12 yr, is explained largely by variation in mortality at HB, and largely by variation in recruitment at SMB. At G and S the relative importance of these two factors appears to have shifted with time. A smaller number of worms blade-1 at SMB than at HB, G or S, may be a consequence of a recruitment limited to the past 1 to 4 yr, which, in turn, may partly explain the interdependence of effects of reef and blade size class on recruitment and mortality. The relationship between level of recruitment and blade base perimeter suggests that the availability of recruits has been consistently high at HB over the past 5 to 12 yr and has increased at G and S in the past 1 to 4 yr. The situation is consistent with a worm population gradually extending northward, dependent on a pool of larvae to the south and limited by a tendency to lose larvae in the north west drift. Because of periodic destruction of tall coral blades in heavy seas, mortality in the seven-spined S. polycerus is, in part, a result of the commensal relationship between the polychaete and the coral. Selection for simultaneous hermaphroditism may be a response to the short life-span of the seven-spined form of S. polycerus and the isolation of breeding individuals resulting from the natural destruction and discontinuous distribution of the host coral.

Upper Silurian lithistid sponge reefs on Somerset Island, Arctic Canada

ABSTRACTThe Upper Ludlow Douro Formation contains the first reported Silurian sponge reefs. These relatively small (5–35 m diameter), mound‐shaped structures contain, on average, 35% lithistid demosponges. Reefs are surrounded by irregular haloes of crinoid debris; abundance and diversity of all fossil groups decreases away from the reefs. Each reef is underlain by a lens of crinoid wackestone to grainstone rich in crinoid holdfasts; trepostomate bryozoans, solenoporacean algae and rhynchonellid brachiopods are locally common. The bulk of each reef consists of lime mudstone with abundant lithistid sponges. This is capped by a thin layer of wackestone with abundant tabulate and rugose corals and fewer lithistid sponges, calcareous algae, trepostomate bryozoans and stromatoporoids. This zonation, in which a sponge colonization community was replaced by a coral diversification community, is similar to that reported from some Middle Ordovician, Upper Jurassic and Holocene sponge reefs.The Douro sponge reefs were relatively low structures, with about 3 m maximum topographic relief. They grew on a broad carbonate platform, probably in warm, tranquil, turbid waters of normal or near‐normal marine salinity. Periodic influxes of terrigenous mud adversely affected reef size, and caused biotic changes. Some of the reef lime mud was derived from non‐reef sources, but significant quantities were also produced on the reefs. Reefs underwent synsedimentary lithification, bioerosion and minor storm erosion. Fabrics and compositions of sparry calcite in cavities record three generations of meteoric cementation. Originally siliceous spicules of the lithistid sponges were dissolved and the moulds later filled with sparry calcite. Early dissolution of siliceous spicules is common in reef environments, and may have caused fossil sponges to be under‐represented in ancient reefs.

Environmental Factors Affecting the Strength of Walleye (Stizostedion vitreum vitreum) Year-Classes in Western Lake Erie, 1960–70

Commercial production of walleyes (Stizostedion vitreum vitreum) from western Lake Erie declined from 5.9 million pounds in 1956 to 140,000 pounds by 1969. Since 1956, marked irregularity in year-class success has developed. Only four year-classes were considered good during 1959–70. The rate and regularity of water warming during the spring spawning and incubation periods in 1960–70 had a positive effect on the density of egg deposits and the resulting year-class strength. Rates of warming were not themselves detrimental, but rather the extended length of the incubation period in cool springs increased the exposure of eggs to such negative influences as dislodgment from the spawning reefs by strong current action generated by spring storms, or siltation and low oxygen tensions. The annual brood stock size had much less influence on year-class strength than did water temperature. Reproductive success was unrelated to fluctuations in size of suitable reef spawning area caused by changes in water level. Apparently the usable spawning area at any water level is more than adequate to serve the limited walleye brood stocks.

Model Study of a Hydraulic Breakwater Over a Reef

Experimental results of model tests of combination hydraulic breakwater and submerged breakwater are given; several sizes of reefs were used in combination with series of hydraulic jet discharges; experimental relationships among reef size, jet discharge and wave characteristics are presented in graphical form.