Burchard et at: Maturity indices and field sampling practices for staging Melanogrammus aeglefinus 
99 
a problem for fecundity studies where ovary weight is 
used as a factor in determining fecundity. For the same 
reason, we also concluded that it is not possible to ac- 
curately stage an ovary as D by macroscopic observa- 
tion alone. This issue poses a problem for studies that 
use gravimetric counting of vitellogenic oocytes and 
oocyte density to determine fecundity. The D stage, 
when the most advanced oocytes in the ovary are in 
the late vitellogenesis phase, is the optimal stage from 
which samples should be taken to determine fecundity. 
Therefore, we recommend that ovary samples be col- 
lected from fishes classified as D on the basis of mac- 
roscopic observations to confirm through microscopic or 
histological analysis that the ovary is in a prespawning 
state. 
Hydration stages 
A challenge in the use of the field index was the subjec- 
tive evaluation of the percentage of hydrated oocytes 
in an ovary that was used to assign the consecutive 
HI, H2, and H3 stages. Therefore, histological samples 
were often assigned to a stage adjacent to the stage 
that was reported in the field. There were 5 instances 
where an ovary was macroscopically classified as HI 
with the field index but microscopically classified as 
the histological stage 3.3. This difference in staging 
was likely due to some variation in individual and tem- 
poral batch fecundity (Trippel et al., 1998). However, 
this error was rare and the hydration stages were cor- 
rectly staged consistently enough that we do not con- 
sider this misclassification problematic in identification 
of the correct hydration stage for the purpose of assess- 
ing diel reproductive patterns. 
The histology-based laboratory staging method un- 
derestimated the HI stage because the ovary typi- 
cally appears to be heterogeneous during this stage 
and, therefore, was not adequately represented in the 
tissue samples. An Hl-classified ovary could be incor- 
rectly identified as D based on histological examination 
under these conditions. However, as an ovary matured 
further, the oocytes appeared to hydrate in unison and 
evenly throughout the ovary and nuclear migration 
and globule yolk coalescence became more evident. 
These criteria reduced the bias in the sampling method 
in later phases of HI and eliminated it in later stages 
H2 and H3. 
Histological analysis verified that H3-stage ova- 
ries were in a state where the next batch of oocytes 
to be spawned were in final OM phase (GVBD), with 
most oocytes fully hydrated. This consistent result is 
important because both the field H3 and histological 
3.3 stages can be confidently used to identify spawning 
readiness, and, therefore, we concluded that they will 
be well suited for use in studies of diel spawning peri- 
odicity in Haddock (Anderson, 2011) and other fishes. 
Ripe and running stage 
When the ovaries of RR females were examined mac- 
roscopically, they exhibited characteristics of the H3 
stage. Furthermore, the tissue samples from these ova- 
ries were classified as 3.3 (SC GVBD; Table 2) with 
histology-based methods. On the basis of results from 
the histological analysis conducted on ovaries classified 
as RR in the field and from the portion of the RR ovary 
full of hydrated oocytes during macroscopic observa- 
tion, we decided to combine the RR and H3 field stages 
into a single stage in the final index (H3; Table 5). 
Use of the RR field stage proved problematic be- 
cause of the sampling method, and we recommend cau- 
tion in its use in future studies. Homans and Vladykoy 
(1954) reported that female Haddock stop feeding dur- 
ing spawning — behavior that would make it difficult 
to catch actively spawning fish with baited gear and 
possibly result in an underestimation of RR females in 
the population. In addition, RR may be overestimated 
because of premature ovulation induced by stress or 
barotrauma. It is hypothesized that the barotrauma 
caused by forcing specimens to ascend to the surface 
from an average depth of 90 m during sampling can 
cause premature ovulation of hydrated oocytes. An 
increased level of cortisol in fishes is an indication of 
severe stress, but it is also involved in the natural pro- 
cess of ovulation (Billard et al., 1981; Wendelaar Bon- 
ga, 1997). The 2-h average soak time of the hooks in 
this study could have been enough time for the stress 
response to induce ovulation in an H3-stage fish before 
it landed on board the fishing vessel. 
For the same reason, histological stage 4.1 may be 
overestimated, because the premature ovulation caused 
by barotrauma results in POFs appearing before they 
normally would. We concluded that it is difficult to 
catch a Haddock in the act of spawning, especially with 
baited hooks; therefore, use of H3-stage fish to estimate 
spawning readiness would be more accurate. However, 
the practice of macroscopically staging a RR Haddock 
through application of pressure to the abdomen and 
observation of the excretion of hydrated oocytes is a 
method that can be used to classify a female as spawn- 
ing ready without need to sacrifice the fish. 
Regressing stage 
The S ovary stage was the most problematic for macro- 
scopic identification. The regressing condition is partic- 
ularly difficult to detect in a species such as Haddock 
with asynchronous development, where batches of eggs 
are spawned multiple times over a prolonged season 
(Hickling and Rutenberg, 1936; West, 1990). Species 
with determinate fecundity complete a spawning sea- 
son by the maturation and spawning of the entire co- 
hort of oocytes developed that year. When only a single 
batch of oocytes was left in the ovary to be spawned, it 
was termed “last spawn.” This stage was evident only 
during histological analysis. Of the ovaries classified 
