Nazlı Demirel

Ichthyoplankton dynamics in a highly urbanized estuary

Abstract Spatio-temporal distribution and composition of ichthyoplankton assemblages were studied in the Golden Horn estuary (Istanbul) over a 10-month period. Environmental parameters were considered to determine the environmental status in different parts (upper, middle and lower) of the estuary. The ichthyoplankton composition of the Golden Horn estuary consisted of 23 species and was dominated by Mullus sp., Diplodus spp. and Liza sp. The largest densities of fish eggs and larvae were found in September 2009 with 786.4 ind. 100 m–3 and 355.9 ind. 100 m−3, respectively. As supported by the multivariate analysis, most species showed a seasonal pattern, with the presence of higher densities during summer and winter. Moreover, the spatial pattern showed that ichthyoplankton distribution and diversity was relatively high in the lower part of the Golden Horn and gradually decreased through the upper parts. Canonical correspondence analysis revealed that spatial changes in depth and water clarity were the main factors forcing larval assemblage distribution and leading to a decrease in density and diversity of fish larvae through the upper part of the estuary. For the seasonal changes, sea surface salinity and chlorophyll a were the main factors in shaping the structure of the larval assemblage and increasing sea surface temperature lead to an increase in the density and diversity of fish larvae.

A new approach for estimating stock status from length frequency data

This study presents a new method (LBB) for the analysis of length frequency data from commercial catches. LBB works for species that grow throughout their lives, such as most commercially-important fish and invertebrates, and requires no input in addition to length frequency data. It estimates asymptotic length, length at first capture, relative natural mortality, and relative fishing mortality. Standard fisheries equations can then be used to approximate current exploited biomass relative to unexploited biomass. In addition, these parameters allow the estimation of length at first capture that would maximize catch and biomass for a given fishing effort, and estimation of a proxy for the relative biomass capable of producing maximum sustainable yields. Relative biomass estimates of LBB were not significantly different from the “true” values in simulated data and were similar to independent estimates from full stock assessments. LBB also presents a new indicator for assessing whether an observed size structure is indicative of a healthy stock. LBB results will obviously be misleading if the length frequency data do not represent the size composition of the exploited size range of the stock or if length frequencies resulting from the interplay of growth and mortality are masked by strong recruitment pulses.