Sea Urchin Populations Research Articles

A Mediterranean rocky-bottom ecosystem fisheries model

An aggregated energy-based simulation model of the Mediterranean infralittoral rocky-bottom was developed and used to carry out fishing experiments where fishing intensity and catch selection were varied. The model is composed of primary producers (algae), herbivores (sea urchins, vagile epifauna and herbivorous fish), suspensivores (sessile epifauna) and carnivores (piscivores and predators of invertebrates). Gross and net primary production are calculated from algal production and respiration parameters. Simulation results suggest that fishing affects the ecosystem's ecology and the benefits of fishing yields must be weighed against changes in the benthic ecology. Many of the changes are indirect effects caused by changes in trophic composition. The model predicts that yields of bony fish can vary by nearly a factor of 250 depending on the types of harvesting restrictions. The model predicts that fish depletion will result in a dominance of sea urchins, which will dramatically reduce algae, epifauna, and gross and net production. When harvesting of sea urchins is added to fisheries, the epifauna reaches high abundance levels once all herbivores have been removed. Maximum equilibrium yields for sea urchins can be quite high (∼850 kg/ha per yr) but sea urchins are easily overfished and show large drops in yields and CPUE for small increases in effort. Because invertivorous fish (i.e. the sparid Diplodus) are the most important part of the finfish fishery, harvesting of sea urchins reduces fish yields and the effort at which maximum yields of fish are achieved by over 70%. Consequently, the value of sea urchins compared with fish is important in determining harvesting policies. Multiple guilds of herbivores keep algal abundance fairly stable over the full range of fishing intensity. Fishing only piscivores will result in a more evenly structured ecosystem, with dominance of herbivorous fish and, in this case, invertivorous fish, control sea urchin populations. Further, algal biomass, gross and net production are high, but yields of fish are minimal. Fishing only invertivores will also result in a dominance of sea urchins, and in a reduction of other fish by competitive exclusion. In this case, maximum fish yields are similar to values when fishing all fish resources, but fewer fishers are supported by the fishery.

Phenotypic plasticity of Paracentrotus lividus (Echinodermata:Echinoidea) in a lagoonal environment

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 152:145-154 (1997) - doi:10.3354/meps152145 Phenotypic plasticity of Paracentrotus lividus (Echinodermata: Echinoidea) in a lagoonal environment Fernandez C, Boudouresque CF The relative sizes of the different somatic and gonad components of the sea urchin Paracentrotus lividus (Lamarck) were studied at 2 sites in a coastal Mediterranean lagoon to demonstrate the phenotypic plasticity of this species to environmental conditions. Sampling sites varied with regard to substrate type, sea urchin population density, and, in particular, food resource. Results indicate that when food resources [beds of Cymodocea nodosa (Ucria) Asherson] are not limiting, P. lividus exhibits high repletion (relative weight of the gut and its content), gonad and test indices. When food is limited, test and gonad indices decline and the relative size of the feeding apparatus increases. These variations may be related to changes in the allocation of energy so as to maximise the acquisition of food. These results show that P. lividus is capable of changing its morphology in response to available food resources and demonstrate the high degree of plasticity of this species. Echinoidea · Paracentrotus lividus · Mediterranean · Lagoon · Phenotypic plasticity · Energy allocation Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 152. Publication date: June 26, 1997 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1997 Inter-Research.

Abalone, sea urchins and rock lobster: a possible ecological shift that may affect traditional fisheries

A possible association between sea urchins Parechinus angulosus and juvenile abalone Haliotis midae has been monitored since 1989 during diving surveys at three sites on the South-Western Cape, South Africa. In 1994, numbers of sea urchins declined dramatically at two sites (Belly's Bay and Mudge Point), with a simultaneous decline in juvenile abalone. This decrease coincided with an apparent influx of rock lobster Jasus lalandii into the kelp beds of these regions. The hypothesis is proposed that predation by rock lobsters caused the previously large population of sea urchins to collapse, resulting in high mortality of juvenile abalone.

Effect of Sea Urchin Reductions on Algae, Coral, and Fish Populations

Many heavily fished coral reefs have a high abundance of sea urchins that may suppress the recovery of fish and coral populations once fishing effort is reduced or eliminated. Restoration of these reefs may be accelerated by intervening and reducing sea urchin populations—particularly those of long‐lived species. We studied three Kenyan coral‐reef lagoons to determine the influence of reductions in sea urchin populations on coral, algae, and fish populations. Populations were monitored seven times over a 1‐year period in 50 × 50 m unmanipulated control plots and experimental sea urchin reduction plots where sea urchin populations were reduced by about 85%. Census of the most‐abundant fish families found the greatest positive population responses to sea urchin reduction in plots protected from fishing. The wet weights of fish nearly tripled, population density increased by 65%, and species richness increased by 30% compared with adjacent control plots. Parrotfish, wrasse, scavenger, and snapper families showed the greatest population and wet‐weight increases. In the two fished reefs, fish populations also increased but to a lesser degree than in the unfished reef—particularly when comparing the wet‐weight estimates of fish. Small‐bodied species of the dam‐selfish and wrasse families and juvenile parrotfish exhibited the largest population increases in these fished reefs. In fished reefs algae and seagrass cover exhibited the greatest increases following sea urchin reduction. Seagrass colonized bare sand, whereas fleshy brown algae colonized hard substrate (dead coral). The tall canopy‐forming alga Sargassum latifolium became the dominant alga in the fished reefs, whereas the more prostrate genus Padina dominated in the unfished reef. Fleshy algae cover in the unfished reef was about half that of the two fished reefs—attributable to the greater abundance of parrotfish in the unfished reef. This suggests that greater herbivory in the unfished reef kept the algae from reaching an algal forest climax typified by Sargassum dominance. In the fished reefs the high S. latifolium cover and the increased thickness of the algal turf reduced hard coral cover by around 30%. In the unfished reef coral cover was reduced by around 13%, but by the end of the experiment coral cover and genera richness were the same in both the control and the sea urchin reduction plots. We conclude that, primarily, fishing and, secondarily and indirectly, high sea urchin abundance are reducing fish numbers and diversity in Kenyan reefs and that sea urchin reduction has the potential to increase reef fisheries production and recovery from overfishing. Sea urchin reduction is not recommended, however, on moderately fished reefs due to the possible loss of coral cover and diversity. If reefs are either severely degraded or if fishing effort is reduced or eliminated, then sea urchin reduction is recommended for the restoration of fish numbers, feeding importance, species diversity, and fisheries production.

Availability and consumption of food by common eiders wintering in the Gulf of St. Lawrence: evidence of prey depletion

To quantify the influence of common eiders (Somateria mollissima) on their food supply in the Gulf of St. Lawrence, eastern Canada, we measured prey availability and patch utilization during two consecutive winters. In this region, eiders fed predominantly near submerged reefs where large populations of blue mussels (Mytilus edulis) and green sea urchins (Strongylocentrotus droebachienensis) were present. Density and biomass of mussels and urchins were estimated for two reefs at the beginning of the winter of 1985 – 1986 using SCUBA and quadrat sampling. Prey selection was assessed by comparing the size of prey from stomach contents of shot eiders with that of prey found in the habitat. Patch use was quantified from elevated blinds by regularly counting eiders from December to April. Mussels in the habitat were small (mean length 6.5 mm) and formed dense beds (mean density 25 398 individuals/m2), whereas urchins were large (mean diameter 35 mm) and occurred at lower densities (114 individuals/m2). However, the average biomass for these two prey species was similar, 2.8 and 3.1 kg wet mass/m2 for mussels and urchins, respectively. The mussels eaten by common eiders tended to be larger than the average size of those available. In contrast, urchins eaten were smaller than the average size present in the habitat. Depending on the reef, consumption of food by eiders ranged from 48 to 69% of the biomass for eiders feeding on mussels and from 3 to 6% of the biomass for urchins. We compared these estimates with those from similar studies and concluded that eiders substantially deplete mussel beds in winter, which in turn seems to affect their distribution.

Factors controlling holocene reef growth: An interdisciplinary approach

Factors controlling holocene reef growth: An interdisciplinary approach

Sea Otters and Kelp Forests in Alaska: Generality and Variation in a Community Ecological Paradigm

Multiscale patterns of spatial and temporal variation in density and population structure were used to evaluate the generality of a three—trophic—level cascade among sea otters (Enhydra lutris), invertebrate herbivores, and macroalgae in Alaska. The paradigm holds that where sea otters occur herbivores are rare and plants are abundant, whereas when sea otters are absent herbivores are relatively common and plants are rare. Spatial patterns were based on 20 randomly placed quadrats at 153 randomly selected sites distributed among five locations with and four locations without sea otters. Both sea urchin and kelp abundance differed significantly among locations with vs. without sea otters in the Aleutian Islands and southeast Alaska. There was little (Aleutian Islands) or no (southeast Alaska) overlap between sites with and without sea otters, in plots of kelp density against urchin biomass. Despite intersite variation in the abundance of kelps and herbivores, these analyses demonstrate that sea otter predation has a predictable and broadly generalizable influence on the structure of Alaskan kelp forests. The percent cover of algal turf and suspension feeder assemblages also differed significantly (although less dramatically) between locations with and without sea otters. Temporal variation in community structure was assessed over periods of from 3 to 15 yr at sites in the Aleutian Islands and southeast Alaska where sea otters were 1) continuously present, 2) continuously absent, or 3) becoming reestablished because of natural range expansion. Kelp and sea urchin abundance remained largely unchanged at most sites where sea otters were continuously present or absent, the one exception being at Torch Bay (southeast Alaska), where kelp abundance varied significantly through time and urchin abundance varied significantly among sites because of episodic and patchy disturbances. In contrast, kelp and sea urchin abundances changed significantly, and in the expected directions, at sites that were being recolonized by sea otters. Sea urchin biomass declined by 50% in the Aleutian Islands and by nearly 100% in southeast Alaska following the spread of sea otters into previously unoccupied habitats. In response to these different rates and magnitudes of urchin reduction by sea otter predation, increases in kelp abundance were abrupt and highly significant in southeast Alaska but much smaller and slower over similar time periods in the Aleutian Islands. The different kelp colonization rates between southeast Alaska and the Aleutian Islands appear to be caused by large—scale differences in echinoid recruitment coupled with size—selective predation by sea otters for larger urchins. The length of urchin jaws (correlated with test diameter, r2 = 0.968) in sea otter scats indicates that sea urchins <15—20 mm test diameter are rarely eaten by foraging sea otters. Sea urchin populations in the Aleutian Islands included high densities of small individuals (<20 mm test diameter) at all sites and during all years sampled, whereas in southeast Alaska similarly sized urchins were absent from most populations during most years. Small (<30—35 mm test diameter) tetracycline—marked urchins in the Aleutian Islands grew at a maximum rate of °10 mm/yr; thus the population must have significant recruitment annually, or at least every several years. In contrast, echinoid recruitment in southeast Alaska was more episodic, with many years to perhaps decades separating significant events. Our findings help explain regional differences in recovery rates of kelp forests following recolonization by sea otters.

Experimental validation of the growth rings of Loxechinus albus (Molina, 1782) in southern Chile (Echinodermata: Echinoidea)

To validate the periodic formation of growth rings in the genital plates of the Chilean edible sea urchin Loxechinus albus (Molina, 1782), an experimental sea urchin population grouped in five size classes (less than 15.0 mm, 20.0–29.9 mm, 50.0–59.9 mm, 70.0–79.9 mm and more than 100.0 mm) was confined to intertidal pools of the Marine Reserve of Mehuin. Size and weight were controlled monthly, from March 1991 to March 1992. At the same time, 50 sea urchins were collected each month from the non-manipulated population, involving the entire range of size distribution present in the study area. From these specimens, the genital plates were taken for ring reading, edge analysis and marginal increase analysis. The data were fitted to the individual growth model of Von Bertalanffy by means of a non-linear method (Marquardt's Algorithm). Asymptotic test diameter ( TD ∞) ranged between 141.2 and 131.7 mm, K (intercept on the TD)-axis) between 0.164 and 0.104 and t 0 (time at age 0; settlement on botttom) between −0.23 and −0.77. In the case of the growth equation for weight, asymptotic weight ( W ∞) ranged between 1207.3 and 740.1 g, and the relationship between test diameter and wet weight was W = 0.00054( TD) 2.94. The regression analysis used for estimating the growth parameters showed no statistically significant differences (95%) between the manipulated and the non-manipulated populations (rings at present and past ages). Moreover, all the growth parameters overlap at the 95% confidence interval obtained from the non-linear fit, showing no statistical differences. On the other hand, a significant relationship was found between the number of plates of a single column of the coronal system and age, which could be useful for the identification of this important demographic parameter in non-destructive field studies.

Status of Kenyan Coral Reefs

The existence of four marine parks and numerous reefs experiencing intense human resource use has provided the opportunity for a number of studies that have helped increase the understanding of human impacts on Kenyan reefs. Studies indicate that the removal of finfish is having the largest impact on unprotected reefs and has a number of secondary and tertiary effects on other faunal groups and ecological processes. A high abundance of sea urchins in unprotected reefs result from reductions in sea urchin predators—largely due to overfishing. High sea urchin populations are associated with reefs with lower coral cover, topographic complexity, and reduced calcium carbonate deposition rates. One reef was converted into a marine park during the study period (1987 to 1994) and showed rapid recovery in coral cover and fish abundance and diversity. Some species of gastropod appear to be affected by shell collecting, but the total fauna seems more affected by removal of their finfish predators. River sediment dis...

Growth and mortality of subtidal red sea urchins (Strongylocentrotus franciscanus) at San Nicolas Island, California, USA: problems with models

Red sea urchins, Strongylocentrotus franciscanus, were tagged with tetracycline in 1990 at subtidal sites off San Nicolas Island, California, USA. After one year in the field, the sea urchins were collected and growth increments were measured based on tetracycline marks, which indicated initial slow growth, a maximum rate, and finally a prolonged period of very slow growth. Small red sea urchins (4 cm diam) were estimated to be 3 to 4 yr old, which is much older than has previously been reported. It is estimated that about 12 yr would be required to attain 10 cm diam. Survival has previously been modeled assuming a constant rate. If the population of red sea urchins is assumed to be stable and stationary, annual survival rate was between 71 and 77% yr-1. Census data for the two years of study have permitted annual survival to be estimated without assuming stable and stationary population structure. Under these conditions, annual survival rate was between 79 and 86% yr-1. Analysis of transitions in the size distributions from 1990 to 1991 suggested that annual survival may have been sizespecific: 91% yr-1 for individuals 1.1 to 4.0 cm diam, 82% yr-1 for individuals 4.1 to 7.0 cm diam, and 63% yr-1 for those of 7.1 to 10.0 cm diam. An alternative explanation to size-specific survival in our study is sizespecific immigration.

Macroparasitic epizootic disease: a potential mechanism for the termination of sea urchin outbreaks in Northern Norway?

Outbreak populations of the green sea urchin, Strongylocentrotus droebachiensis Muller, in Vestfjorden, Northern Norway, are infected by the endoparasitic nematode, Echinomermella matsi Jones and Hagen, 1987. The prevalence of E. matsi has increased from 5.5% in 1983 to 65.4% in 1991 at a study site in Godoystraumen. Infected sea urchins had a lower mean gonad index than non-infected sea urchins, and two thirds of the infected sea urchins were reduced to virtual castrates of unknown sex. The mean density of the S. droebachiensis-population in Godoystraumen appeared unchanged, but the mean test diameter had decreased by 14.6%, from 36.4 mm in 1983 to 31.1 mm in 1991. As a result of these changes the reproductive capacity of the sea urchin population appears to have been reduced by approximately 58%. The data analysis was augmented by the use of a PEPLOM (PErcentile PLOt Matrix), which appears to be a novel combination of two powerful methods of graphical data analysis, i.e., the percentile comparison graph and the scatterplot matrix (SPLOM). Changes in the size-frequency distribution, and in the size-dependent pattern of parasite infection in the S. droebachiensis-population in Godoystraumen, are consistent with a hypothesis of increased parasite-related mortality. The larger hypothesis, that E. matsi may function as a macroparasitic terminator of sea urchin outbreaks in northern Norway, could not be rejected.

Functional response of the predators American lobster Homarus americanus (Milne-Edwards) and Atlantic wolffish Anarhichas lupus (L.) to increasing numbers of the green sea urchin Strongylocentrotus droebachiensis (Müller)

The predatory impact of the American lobster Homarus americanus and the Atlantic wolffish Anarhichas lupus on the green sea urchin Strongylocentrotus droebachiensis was investigated in a multifactorial experiment. Both predators exhibited either Type 2 or Type 3 functional responses to increasing prey density, but the magnitude of the response differed for the two predators, and for different prey sizes. Predation on large (>20 mm) sea urchins increased approximately three-fold, i.e., from 1.12 to 3.54 urchins·day −1 for wolffish, and from 0.35 to 0.97 urchins·day −1 for lobster, when prey density was increased sixfold, i.e., from 5 to 30 urchins·tank −1. Although more urchins were killed at the highest density, the proportion of available urchins that were killed dropped by ≈50%. Predation on small (⪯20m) sea urchins was similar for both predators, increasing from ≈0.5 to ≈1.9 urchins·day −1 with increased prey density. In the lobster treatment, a rock crab, Cancer irroratus Say, was included as alternative prey. The lobsters killed 5.78% of the total number of available rock crabs, and 5.01% of the total number of available urchins, but lobster predation on urchins was nearly halved in replicates where the crab was killed. Experimental factors other than prey density and prey size, i.e., season; availability of food and physical refuges for the prey, and whether the prey had been fed or starved prior to the experiment , had no significant effects on the predation rates of either predator. These results are consistent with the hypothesis that green sea urchin outbreaks may be triggered by reductions in predation pressure. There was no evidence of significantly increased predation on morbid sea urchins although ≈1% of the experimental sea urchin population exhibited symptoms of amoeboid, Paramoeba invadens (Jones), disease.

Kenyan coral reef-associated gastropod assemblages: distribution and diversity patterns

A survey of Kenya's shallow water (<2 m) coral reef-associated prosobranch fauna was undertaken to determine patterns of distribution, density, diversity and species richness, and the possible role of other reef fauna and human utilization on these patterns. The sample assemblage of 135 species from 25 families is similar to other Indian Ocean regions with no apparent endemism or subregional faunal affinities. Species richness, determined by species-individual relationships, has been reduced by approximately 45% since the Pleistocene. Northern Kenya, typified by small coral islands experiencing river and estuarine discharges had low densities and species richness and high species variability. This is attributable to the interrelated factors of river discharge, small reefs and reduced predator refuge. Southern Kenya's more expansive fringing reef has a denser and richer fauna but appears less species rich than Tanzania. Variation within reefs suggests similarities in diversity between reef lagoons, flats and edges, but lagoons had lower densities than reef flat or edge sites. This is attributable to greater predation rates within lagoons. Species composition between reef locations was variable but differed for comparisons between reef lagoons and reef flats. The population densities of thirty commercially collected species were compared between shelled and unshelled reefs. Only two commercial strombids, Lambis truncata and L. chiragra, had lower densities within shelled compared to unshelled reefs. Within six southern Kenvan reef lagoons, total gastropod densities were negatively correlated with the Balistidae (triggerfish) and total fish densities and positively with sea urchin densities. The removal of balistids through fishing appears to lead to co-occurring population increases in gastropod and sea urchin populations which, in most instances, appears to negate the effect of shell collecting.

Causes and consequences of sea urchin abundance and diversity in Kenyan coral reef lagoons.

Large differences in community structure of sea urchins and finfish have been observed in Kenyan reef lagoons. Differences have been attributed to removal of finfish predators through human fishing activities. This study attempted to determine (i) the major sea urchin finfish predators, (ii) the effect of predation on sea-urchin community structure, and (iii) the possible effect of sea urchin increases and finfish decreases on the lagoonal substrate. Six reefs, two protected and four unprotected, were compared for differences in finfish abundance, sea urchin abundance and diversity and substrate cover, diversity and complexity. Comparisons between protected and unprotected reefs indicated that finfish populations were ca. 4 x denser in protected than unprotected reefs. Sea urchin populations were >100 x denser and predation rates on a sea urchin, Echinometra mathaei, were 4 x lower in unprotected than in protected reefs. The balistidae (triggerfish) was the single sea-urchin finfish predator family which had a higher population density in protected than in unprotected reefs. Balistid density was positively correlated with predation rates on tethered E. mathaei (r=0.88; p<0.025) and negatively correlated with total sea-urchin density (r=-0.89; p<0.025) on the six reefs. We conclude from observations that the balistids Balistaphus undulatus and Rhinecanthus aculeatus are the dominant sea-urchin predators. The sea-urchin assemblage had its greatest diversity and species richness at intermediate predation rates and low to intermediate sea-urchin densities. At low predation rates and high sea-urchin density E. mathaei dominated the assemblage's species composition. Preferential predation on the competitive dominant maintains the assemblage's diversity, supporting the compensatory mortality hypothesis (Connell 1978) of coral reef diversity. Protected reefs had greater cover of hard coral, calcareous and coralline algae, and greater substrate diversity and topographic complexity than unprotected reefs which had greater algal turf and sponge cover. Coral cover and topographic complexity were negatively correlated with total sea urchin density. Although experimentation is lacking, these substrate changes may be due to the switch from finfish to sea-urchins as consumers which results from overfishing of finfish. Removal of top invertebrate-eating carnivores appears to have cascading effects on the entire coral reef ecosystem.

Disturbance and organization of macroalgal assemblages in the Northwest Atlantic

Large seaweeds are often structurally dominant in subtidal and intertidal rocky shore benthic communities of the N.W. Atlantic. The mechanisms by which these algal assemblages are maintained are surprisingly different in the two habitats. In the subtidal community, kelps are dominant space competitors in the absence of strong grazing interactions. In contrast, the large perennial seaweeds of intertidal zones (fucoids and Chondrus crispus) are competitively inferior to both sessile filter feeders and ephemeral, pioneer algal species. Intertidal seaweed beds are maintained by carnivory of whelks, which reduces filter feeder populations, and by herbivorous periwinkles which reduce ephemeral algal populations. Through most of the intertidal zone, disturbance, both biological and physical, dictates which species shall compete and equilibrium conditions obtain subsequently. The roles of subtidal consumers are quite different. Sea urchins are the major algal herbivores and these voracious animals maintain an equilibrium state in which large tracts of subtidal coralline pavement are kept free of kelp forests. Urchins do not seem to play a successional facilitative role for kelps in the way that periwinkles do for fucoids in the intertidal. Control of herbivore populations is thus a key to the maintenance of subtidal foliose algal beds. It is clear that parasitic amoebas can decimate sea urchin populations so that kelp forest dominance is assured. However, the importance of carnivory in limiting urchins in the subtidal community is unclear in the absence of appropriate manipulation experiments. It is possible that carnivorous decapods and fin fish control sea urchin populations and hence foliose algal abundance, but this must remain speculative. The seaweed-dominated state of the subtidal system is an alternative equilibrium condition to the urchin/coralline alga configuration. The structure of the kelp beds is relatively uniform in responding to frequent small-scale, infrequent large-scale, or no, disturbance.

Heat waves, baby booms, and the destruction of kelp beds by sea urchins

Large populations of sea urchins, Strongylocentrotus droebachiensis (Muller), destroyed kelp beds along the Atlantic coast of Nova Scotia in the 1960's and 1970's. The origin of these large sea urchin populations is not understood. We have investigated the potential influence of variable growth and development of the planktonic larvae of sea urchins (in response to temperature and food abundance) on recruitment of benthic juveniles. The adult sea urchins were collected at Sandy Cove, Digby County, Nova Scotia, Canada, in December 1986. Temperature strongly affected larval size and the growth of the echinus rudiment within the range 3° to 9°C, and larvae grew most rapidly at 14°C. Food abundance had a smaller effect on larval growth, and these effects were apparent only at high temperature. Larvae fed the same concentration of two different algal food species grew and developed similarly. Correspondence between spring temperature variation and qualitative variation in sea urchin recruitment, as well as strong temperature effects on larval growth in culture, and the occurrence of a large, positive temperature anomaly in June 1960, all suggest that temperature effects on larval growth and development may have led to intense sea urchin recruitment in 1960 and the appearance of large adult populations 4 to 6 yr later. This result invites further research.

Increased macroalgal abundance following mass mortalities of sea urchins (Strongylocentrotus droebachiensis) along the Atlantic coast of Nova Scotia.

Recurrent outbreaks of disease between 1980 and 1983 caused catastrophic mortality of sea urchins (>260,000 t fresh weight) along 280 km (straight line distance) of the Atlantic coast of Nova Scotia. The complete elimination of sea urchins and concomitant development of fleshy macroalgal communities have occurred along different parts of this coast in different years. Macroalgal communities in areas where sea urchins died off 1, 3 and 4 years previously are compared to existing sea urchin-dominated barren grounds and to a mature kelp bed without sea urchins. Changes in macroalgal cover and species composition, and increases in biomass, density and size of kelp (Laminaria) species, characterize the succession from barren grounds to 3- and 4-year-old kelp beds. The greatest change occurred between one and three years following sea urchin mass mortality. Within 3 years, kelp beds attained a level of biomass (7.6 kg m-2) comparable to that of mature beds. Recovery of sea urchin populations via recruitment of planktonic larvae has been slow and spatially variable. Large-scale reciprocal fluctuations in kelp and sea urchin biomass may characterize the trajectory of a dynamic system which cycles between two alternate community states: kelp beds and sea urchin-dominated barren grounds. Periodic decimation of sea urchin populations by disease may be an important mechanism underlying this cyclicity.

Sensitivity of fitness to macroparameter changes: an analysis of survivorship and individual growth in sea urchin life histories.

An analysis for an age-structured demography was conducted to explore the sensitivity of fitness to individual parameters of a growth equation (Brody-Bertalanffy) and a survivorship function (Weibull). Initial parameters were selected to simulate populations of sea urchins and include northern and southern populations of the temperate sea urchin Strongylocentrotus purpuratus and tropical species such as Diadema antillarum. Results of the analysis are intuitively reasonable. Regardless of whether a population is increasing or decreasing, there would always be an increase in fitness associated with increases of: 1) the growth-rate constant, 2) maximum gonad size, 3) maximum lifespan, 4) the shape parameter of the Weibull function; and, decreases of 1) the age at first reproduction and 2) the ratio of number of recruits/spawn weight. If trade-offs are possible then improved survival characteristics and increased lifespan would be favored in declining populations. In general, relative sensitivity is dependent upon which parameter is varied in the simulations. The analysis shows which changes of parameters would bring about the greatest change in fitness but can not show trade-offs or design constraints. An example of trade-off in urchin life history is the relationship between the growth-rate constant, K and maximum lifespan, ω. A doubling of lifespan is associated with a halving of K.

Succession in sea urchin and seaweed abundance in Nova Scotia, Canada

From 1980 through 1983 sea urchin mass mortalities were at least 245 000 t. The habitat released to seaweed is expected to support a standing crop of 1.8 million tons and an annual production of 7 million tons. An extensive survey of 2 900 km of Nova Scotia shoreline revealed that 511 km2 of habitat (rock bottom less than 15 m deep) was available to seaweeds. Areas most exposed to ocean swells bordered 26% of the shoreline length but included 82% of the seaweed habitat. Before mass mortalities, seaweed refuges from sea urchin grazing included very sheltered habitats, very exposed habitats, and boulders set among shifting sand; however, these were less than 10% of the total habitat. In the 1 to 2 years following mass mortalities seaweed cover and occurrence increased significantly for all of five categories of wave exposure and for six of seven algal taxa respectively. In at least three locations the sea urchin population began to recover as a result of larval recruitment but in 1983 was again reduced by disease. Sea urchins appear to lack natural resistance to disease, at least at warm temperatures. In the laboratory all sea urchins collected from the Bay of Fundy, southern Nova Scotia, eastern Newfoundland, and an area recovering from mass mortalities died following exposure to diseased animals. Also, field mortalities included a wide variety of habitats. Interviews of Nova Scotia lobster fishermen revealed that sea urchin mass mortalities have probably occurred before in this century, but infrequently.

Spiny lobsters and sea urchins: Analysis of a predator-prey interaction

Spiny lobsters, Panulirus interruptus (Randall), are important predators of two species of sea urchins, Strongylocentrotus franciscanus (A. Agassiz) and S. purpuratus (Stimpson), which are major herbivores in southern California kelp forest communities. Aquarium experiments indicate that spiny lobsters strongly prefer S. purpuratus over S. franciscanus of comparable test diameter, probably accounting for the higher number of S. purpuratus mortalities observed in the field. Juvenile S. franciscanus (< 60 mm test diameter, TD) are preferred over larger conspecifics, but the smallest S. franciscanus (< 30 mm TD) are protected by association with the adult urchin spine canopy. Mid-sized S. franciscanus too large to fit under the spine canopy are subject to intense predation. All sizes of P. interruptus tested showed a notable reluctance to attack large S. franciscanus (> 90 mm TD) when offered a range of sizes, although the larger lobsters are capable of handling the largest urchins. Relative movement, spine length, test thickness, and gonad weights of sea urchins were examined to evaluate the factors affecting the lobsters' choice of prey, and behavioral responses to lobsters were assessed to determine urchin availability to these predators. The handling problems associated with the long spines of larger S. franciscanus appear to be the major considerations in prey choice despite the much greater potential energy gain from these urchins. Thus predation by spiny lobsters appears to be sufficient to explain the bimodal size-frequency distribution typical of S. franciscanus populations in areas where these predators are common. Heavy fishing pressure on spiny lobsters during this century probably resulted in the release of sea urchin populations and contributed to episodes of destructive urchin grazing observed in the 1950's and subsequent years.