310 
Fishery Bulletin 108(3) 
Mean trophic ievel The position of a stock within the 
larger fish community can be used to infer stock pro- 
ductivity; lower-trophic-level stocks are generally more 
productive than higher-trophic-level stocks. The trophic 
level of a stock can be computed as a function of the tro- 
phic levels of the organisms in its diet. For this attribute, 
stocks with trophic levels higher than 3.5 were catego- 
rized as low-productivity stocks and stocks with trophic 
levels less than 2.5 were categorized as high-productiv- 
ity stocks, and moderate-productivity stocks would fall 
between these bounds. These scoring thresholds roughly 
categorize piscivores to higher trophic levels, omnivores 
to intermediate trophic levels, and planktivores to lower 
trophic levels (Pauly et al., 1998) and carry the assump- 
tion that the food web analysis did not consider microbial 
loops as an individual trophic level. 
Susceptibility attributes 
Previous applications have been focused on the catch- 
ability and mortality of stocks, and other attributes, 
such as management effectiveness and effects of fish- 
ing gear on habitat quality, have been addressed in 
subsequent analyses (Hobday et al. 2 ). Our susceptibil- 
ity index includes all these attributes in an effort to 
make the results of our analysis more transparent and 
understandable. We defined 12 susceptibility attributes; 
the first seven relate to catchability and the other five 
measure management factors. 
Like the susceptibility attributes of Hobday et al. 2 , 
catchability attributes provide information on the likeli- 
hood of a stock’s capture by a particular fishery, given 
the stock’s range, habitat preferences, behavioral re- 
sponses, and morphological characteristics that may 
affect its susceptibility to the fishing gear deployed in 
that fishery. For management attributes, one must con- 
sider how the fishery is managed: for example, fisheries 
with conservative management measures in place that 
effectively control the amount of catch are less likely to 
overfish. For some of these attributes, the criteria are 
somewhat general in order to accommodate the wide 
range of fisheries and management systems. 
Areal overlap This attribute pertains to the extent of 
geographic overlap between the known distribution of a 
stock and the distribution of the fishery. Greater over- 
lap implies greater susceptibility, because some degree 
of geographical overlap is necessary for a fishery to 
impact a stock. The simplest approach to determining 
areal overlap is to evaluate, either qualitatively or 
quantitatively, the proportion of the spatial distribu- 
tion of a given stock that overlaps that of the fishery, 
based on known geographical distributions of both. 
Geographic concentration Geographic concentration 
is the extent to which the stock is concentrated into 
small areas. We included this attribute because a stock 
with a relatively even distribution across its range may 
be less susceptible than a highly aggregated stock. For 
some species, a useful measure of this attribute is the 
proportion of an area of interest occupied by a specified 
percentage of the stock (Swain and Sinclair, 1994), which 
can be computed if survey data exist (see Patrick et al., 
2009). For many stocks, this measure gives a general 
index of areal coverage that relates well to geographic 
concentration. However, some stocks can be concentrated 
in a small number of locations throughout a survey 
area (i.e., a “patchy” stock that is distributed over the 
survey area). Thus, some refinements to the index may 
be necessary to characterize geographic concentration 
in these cases. 
Vertical overlap Like geographic overlap, this attribute 
concerns the position of the stock within the water 
column (e.g., demersal or pelagic) in relation to the 
fishing gear. Information on the depth at which gear is 
deployed (e.g., depth range of hooks for a pelagic longline 
fishery) and the depth preference of the species (e.g., 
obtained from archival tagging or other sources) can be 
used to estimate the degree of vertical overlap between 
fishing gear and a stock. 
Seasonal migrations Seasonal migrations either to or 
from the fishery area (i.e., spawning or feeding migra- 
tions) could affect the overlap between the stock and the 
fishery. This attribute also pertains to cases where the 
location of the fishery changes seasonally, and therefore 
may be relevant for stocks captured as bycatch. 
Schooling, aggregation, and other behaviors This attri- 
bute encompasses behavioral responses of both individ- 
ual fish and the stock in response to fishing. Individual 
responses may include, for example, herding or gear- 
avoidance behavior that would affect catchability. An 
example of a population-level response is a reduction in 
the area of stock distribution with reduction in popula- 
tion size, potentially leading to increases in catchability 
(MacCall, 1990). 
Morphological characteristics affecting capture This 
attribute pertains to the ability of the fishing gear to 
capture fish according to their morphological character- 
istics (e.g., body shape, spiny versus soft rayed fins). On 
a population level, this attribute refers to gear selectivity 
as it varies with fish size and age. Scoring this attribute, 
one should take into consideration what portion of the 
population size or age composition is accessible to the 
fishing gear or gears in question. Particular attention 
should be paid to the size or age at maturity in relation 
to capture. 
Desirability or value of the fishery For this attribute, 
one assumes that highly valued fish stocks are more 
susceptible to overfishing or becoming overfished 
by recreational or commercial fishermen because of 
increased fishing effort. To identify the value of the 
fish, we used the price per pound or annual landings 
value for commercial stocks (using the higher of the 
two values; see Table 2) or the retention rates for rec- 
reational fisheries. 
