Ta. Ebert et al., INFERRING DEMOGRAPHIC-PROCESSES FROM SIZE-FREQUENCY DISTRIBUTIONS - EFFECT OF PULSED RECRUITMENT ON SIMPLE-MODELS, Fishery bulletin, 91(2), 1993, pp. 237-243
The shape of a size-frequency distribution is the result of age- or si
ze-specific rates of growth and survival, their variability, and seaso
nal and interannual variation in recruitment. Simulation of size distr
ibutions can be used to gain insight into the underlying processes tha
t give rise to observed size structure of organisms in the field, but
the utility of this approach depends critically on underlying assumpti
ons. Incorrect judgment of the significance of assumptions can lead to
erroneous conclusions concerning the causes of bi- or polymodal distr
ibutions. Using the Brody-Bertalanffy growth model and a constant surv
ival rate, bi- and polymodal distributions can be generated when recru
itment is pulsed. Even with as many as 10 recruitment episodes per yea
r, size distributions show several modes. A sampling of the literature
indicates that most fish and marine invertebrates have pulsed rather
than continuous recruitment; thus, when very little is known about a s
pecies, pulsed rather than continuous recruitment would be the better
assumption when interpreting the shapes of size distributions. Our sim
ulations differ from those conducted by Barry & Tegner (1990) who assu
med continuous and constant recruitment and focused on changing growth
and survival parameters to explain bimodal size structure. These auth
ors also suggested that their analysis was appropriate for interpretin
g the dynamics of red sea urchins Strongylocentrotus franciscanus. We
have been documenting settlement of both red and purple (S. purpuratus
) sea urchins. At La Jolla, California, neither species showed continu
ous settlement; rather, both species had pulses of settlement in sprin
g 1990 and 1991. Although age-specific variation in growth or mortalit
y parameters can result in bimodal size distributions, it is more like
ly that such distributions are caused by seasonal pulses of recruitmen
t.