Fishery independent indices of spawning biomass of Atlantic bluefin tuna in
western North Atlantic Ocean are presented which utilize National Marine
Fisheries Service ichthyoplankton survey data collected from 1977 through
2007 in the Gulf of Mexico. Indices were developed using similarly
standardized data from which previous indices were developed (i.e. abundance
of larvae with a first daily otolith increment formed per 100 m2 of
water sampled with bongo gear). Indices were also developed for the first
time from standardized data collected with neuston gear [i.e. abundance of
5-mm larvae (i.e. seven-day-old larvae) per 10 minute tow]. Indices of
larval abundance were developed using delta-lognormal models, including
following covariates: time of day, time of month, area sampled and year. Due
to the large frequency of zero catches during ichthyoplankton surveys, a
zero-inflated delta-lognormal approach was also used to develop indices.
Finally, a multivariate delta-lognormal approach was employed to develop
indices of annual abundance based on both bongo and neuston catches. The
results of these approaches were compared with one another and with other
indices of larval abundance previously developed for the Gulf of Mexico.
Residual analyses indicated that abundance indices of Atlantic bluefin tuna
larvae were more appropriately developed from bongo-collected data through
the zero-inflated delta-lognormal approach than other data sets and modeling
approaches. Also, when modeling bongo-collected data with the zero-inflated
delta-lognormal approach, the index values increased, indicating some
correction for zero-inflation, and their variability decreased as compared
to indices developed with the delta-lognormal approach.