General Trends and Interannual 

Variability in Prey Selection by 

Larval Cod and Haddock from the 

Southern Flank of Georges Bank, 

May 1993-1999 

US DEPARTMENT OF COMMERCE 

National Oceanic and Atmospheric Administration 

National Marine Fisheries Service 

Northeast Fisheries Science Center 

Woods Hole, Massachusetts 

May 2010

NOAA Technical Memorandum NMFS-NE-217 
Recent Issues in This Series:

198. Essential Fish Habitat Source Document: Bluefish, Pomatomus saltatrix, Life History and Habitat Characteristics. 2nd ed. 

By Jon K.T. Brodziak. December 2005. vi + 89 p., 48 figs., 5 tables, 1 app. NTIS Access. No. PB2006-103439. [Online publication 

only.]

199. Distribution and Abundance of Fish Eggs Collected during the GLOBEC Broad-Scale Georges Bank Surveys, 1995-1999. 

By John D. Sibunka, Donna L. Johnson, and Peter L. Berrien. August 2006. iv + 72 p., 28 figs., 1 table. NTIS Access. No. PB2008-

107379. [Online publication only.] 

200. Essential Fish Habitat Source Document: Black Sea Bass, Centropristis striata, Life History and Habitat Characteristics (2nd 

ed. By Amy F. Drohan, John P. Manderson, and David B. Packer. February 2007. vi + 68 p., 33 figs., 2 tables. NTIS Access No. 

PB2008-107378. [Online publication only.]

201. U.S. Atlantic and Gulf of Mexico Marine Mammal Stock Assessments -- 2006. By Gordon T. Waring, Elizabeth Josephson, 

Carol P. Fairfield, and Katherine Maze-Foley, eds. Dana Belden, Timothy V.N. Cole, Lance P. Garrison, Keith D. Mullin, Christopher 

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202. Evaluation of Northern Right Whale Ship Strike Reduction Measures in the Great South Channel of Massachusetts. By RL 

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203. Essential fish habitat source document: Spiny dogfish, Squalus acanthias, life history and habitat characteristics, 2nd 

edition. By LL Stehlik. December 2007. NTIS Access No. PB2008-107376. 

204. An Evaluation of the Northeast Region’s Study Fleet pilot program and Electronic Logbook System: Phases I and II. By 

Michael C. Palmer, Susan E. Wigley, John J. Hoey, and Joan E. Palmer. December 2007. NTIS Access No PB2008-107374. 

205. U.S. Atlantic and Gulf of Mexico Marine Mammal Stock Assessments -- 2007. By GT Waring, E Josephson, CP Fairfield, and 

K Maze-Foley, Editors. November 2007. NTIS Access No _______. 

206. Growth of Black Sea Bass (Centropristis striata) in Recirculating Aquaculture Systems. By Dean M. Perry, David A. Nelson, 

Dylan H. Redman, Stephan Metzler, and Robin Katersky. October 2007. NTIS Access No. PB2008-107374. 

207. Analysis of Atlantic Sea Scallop (Placopecten magellanicus) Fishery Impacts on the North Atlantic Population of 

Loggerhead Sea Turtles (Caretta caretta). By Richard Merrick and Heather Haas. February 2008. NTIS Access No PB2008-107373. 

208. Global Applications of the Large Marine Ecosystem Concept 2007-2010. By Kenneth Sherman, Marie-Chirstine Aquarone, and 

Sally Adams. June 2007. NTIS Access No. PB______-_______. 

209. Impacts to Marine Fisheries Habitat from Nonfishing Activities in the Northeastern United States. By Michael R. Johnson, 

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210. U.S. Atlantic and Gulf of Mexico Marine Mammal Stock Assessments -- 2008. By GT Waring, E Josephson, CP Fairfield, and 

K Maze-Foley, Editors. February 2009. NTIS Access No _______. 

211. Trends in Selected Northeast Region Marine Industries. By Eric M. Thunberg. July 2008. NTIS Access No. 

212. Summary of harbor porpoise (Phocoena phocoena) bycatch and levels of compliance in the northeast and mid–Atlantic 

gillnet fisheries after the implementation of the Take Reduction Plan: 1 January 1999 – 31 May 2007. By Debra Palka, Christopher 

D. Orphanides, and Melissa L. Warden. August 2008. NTIS Access No. 

213. U.S. Atlantic and Gulf of Mexico Marine Mammal Stock Assessments -- 2009. By Gordon T. Waring, Elizabeth Josephson, 

Katherine Maze-Foley, and Patricia E. Rosel. December 2009. NTIS Access No. 

214. Mortality and Serious Injury Determinations for Baleen Whale Stocks along the United States and Canadian Eastern 

Seaboards, 2004-2008. By Allison H. Glass, Timothy V.N. Cole, and Mendy Garron. May 2010.NTIS Access No. 

215. Atlantic Sturgeon Research Techniques. By Kimberly Damon-Randall, Russell Bohl, Stephania Bolden, Dewayne Fox, Christian 

Hager, Brian Hickson, Eric Hilton, Jerre Mohler, Erika Robbins, Tom Savoy, Albert Spells. May 2010. NTIS Access No. 

216. The Trophic Dynamics of 50 Finfish and 2 Squid Species on the Northeast US Continental Shelf. By Brian E. Smith and Jason 

S. Link. May 2010. NTIS Access No.
General Trends and Interannual Variability 

in Prey Selection by Larval Cod and 

Haddock from the Southern Flank of 

Georges Bank, May 1993-1999 

Elisabeth A. Broughton and R. Gregory Lough 

National Marine Fisheries Service, Northeast Fisheries Science Center 

166 Water St, Woods Hole, MA, 02543 

US DEPARTMENT OF COMMERCE 

Gary Locke, Secretary 

National Oceanic and Atmospheric Administration 

Dr. Jane Lubchenco, Administrator 

National Marine Fisheries Service 

Eric C. Schwaab, Assistant Administrator for Fisheries 

Northeast Fisheries Science Center 

Woods Hole, Massachusetts 

May 2010

NOAA Technical Memorandum NMFS-NE-217 

This series represents a secondary level of scientific publishing. All issues employ thorough internal 

scientific review; some issues employ external scientific review. Reviews are transparent collegial 

reviews, not anonymous peer reviews. All issues may be cited in formal scientific communications. 
Editorial Notes 

Information Quality Act Compliance: In accordance with section 515 of Public Law 106-554, the 

Northeast Fisheries Science Center completed both technical and policy reviews for this report. 

These predissemination reviews are on file at the NEFSC Editorial Office. 

Species Names: The NEFSC Editorial Office’s policy on the use of species names in all 

technical communications is generally to follow the American Fisheries Society’s lists of 

scientific and common names for fishes, mollusks, and decapod crustaceans and to follow the 

Society for Marine Mammalogy's guidance on scientific and common names for marine 

mammals. Exceptions to this policy occur when there are subsequent compelling revisions in the 

classifications of species, resulting in changes in the names of species. 

Statistical Terms: The NEFSC Editorial Office’s policy on the use of statistical terms in all 

technical communications is generally to follow the International Standards Organization’s 

handbook of statistical methods. 

Internet Availability: This issue of the NOAA Technical Memorandum NMFS-NE series is 

being as a Web document in HTML (and thus searchable) and PDF formats and can be 

accessed at: http://www.nefsc.noaa.gov/nefsc/publications/. 
1 

TABLE OF CONTENTS 

ABSTRACT .................................................................................................................................... 2 

INTRODUCTION .......................................................................................................................... 3 

MATERIALS AND METHODS .................................................................................................... 3 

RESULTS ....................................................................................................................................... 5 

DISCUSSION ................................................................................................................................. 8 

ACKNOWLEDGEMENTS .......................................................................................................... 11 

REFERENCES CITED ................................................................................................................. 12 

APPENDIX ................................................................................................................................... 32 
2 

ABSTRACT 

 Stomach samples were examined from 1080 larval cod (Gadus morhua) and 2586 larval 

haddock (Melanogrammus aeglefinus) collected in May of 1993, 1994, 1995, 1997, and 1999 

from mixed and stratified waters on the southern flank of Georges Bank. Larvae were grouped 

into 3 size classes: 3-5 mm, 6-8 mm, and 9-13 mm. Prey were grouped into 28 species and life 

stage based categories. Copepods comprised 98% of prey consumed. Individual years could be 

categorized into three groups: 1993-94, 1995 and 1999, and 1997. In 1993-1994, smaller larvae 

ingested Pseudocalanus spp. nauplii, adding gravid female Pseudocalanus spp. in larvae larger 

than 5 mm standard length (SL). In 1995 and 1999, larvae ingested similar prey numbers and had 

the same range of total stomach biomass as in 1993-94 but selected a wider range sizes and 

copepod species. In 1997, larvae consumed comparable numbers of Pseudocalanus spp. but 

mean prey size and total stomach biomass were lower than in other years. Chesson's α values for 

each larval size class showed positive selection for all life stages of Pseudocalanus spp. and 

varying life stages of Oithona spp. Calanus finmarchicus was rarely selected by any larvae. As 

larval length increased, stomach biomass increased and larger prey were selected. An analysis of 

niche overlap showed cod and haddock diets had high degrees of overlap. Individual years 

showed a high degree of overlap of the species of larval prey between the mixed and stratified 

sites. Combining larvae from all years showed significant differences in mean prey count, prey 

length, and stomach biomass between larvae taken from the mixed and stratified sites. Feeding at 

the mixed site did not vary with depth, but larval feeding was affected by depth at the stratified 

site. All larvae showed a diel feeling pattern with feeding increasing soon after dawn and 

decreasing after dusk. 
3 

INTRODUCTION 

 Larval fish feeding patterns are an important component of early life history studies, 

recruitment studies, and more recently, coupled physical-biological growth models and 

ecosystem based fishery management plans. Understanding factors that influence interannual 

variations in prey selection and general feeding patterns has the potential to contribute to 

defining essential fish habitat as well as increasing model and recruitment prediction accuracy. 

There is a general hypothesis that favorable prey conditions lead to rapid growth which reduces 

the larval period resulting in greater survival and recruitment (Lough et al. 2005). Prior larval 

gadid feeding studies agree that early larvae prey predominately on calanoid copepod nauplii 

progressing to copepodites and smaller copepod adults as they get larger (Heath and Lough 

2007). However, prior studies have also shown significant variation in the copepod species 

selected and additional noncopepod items included in the diet. Mayer and Wahl (1977) noted 

larval feeding was affected by a complex mix of size and taxa preferences creating an apparent 

lack of consistent patterns in larval prey selectivity. On Georges Bank, larval gadid feeding 

patterns have been inferred from single year studies (Marak 1960; Sherman et al. 1981; Kane 

1984; Lough et al. 1996) or a data set which combined years but did not differentiate between 

them (Auditore et al. 1994; Lough and Mountain 1996). While single year studies provide insight 

about factors influencing larval diet for that year, they provide incomplete information on factors 

that may consistently impact larval feeding patterns. The Northwest Atlantic Global Ocean 

Ecosystem Dynamics (GLOBEC) process studies offered a unique opportunity to collect a 

multiyear, geographically similar, depth stratified, larval gadid feeding and prey field database. 

Information was collected on the size, species, and life stage for all larval gadid prey. This 

database currently provides several individual based modeling studies with prey fields, feeding 

intensity curves, and field data used to compare modeled results with in situ feeding (Petric et al. 

2009; Kristiansen et al. 2009; Lough et al. 2005,2006). This technical memorandum presents an 

overview of five years of prey field composition, oceanographic conditions, and larval gadid 

species based feeding. 

MATERIALS AND METHODS 

 Plankton samples were collected in May from 1993-1999 along the southern flank of 

Georges Bank as part of the process studies of the Northwest Atlantic GLOBEC program (Wiebe 

et al. 2003). Each cruise first conducted an east to west grid of transects spaced 5nm apart along 

the southern flank. Standard Marine Resources Monitoring, Assessment, and Prediction 

(MARMAP) sampling (Jossi and Marak 1983) was conducted with a 61cm bongo equipped with 

333μm and 505μm mesh nets. Ichthyoplankton from the 505μm net were sorted and identified at 

sea to establish areas within the grid containing high concentrations of gadid larvae. Two 

sampling sites were chosen each year that represented well-mixed and stratified water column 

conditions (Fig. 1). Ichthyoplankton were collected with a 1-m2

 Multiple Opening Closing Net 

Environmental Sensing System (MOCNESS) equipped with nine 333μm mesh nets opened at 10 

m depth intervals. The prey field was concurrently sampled with a 1⁄4-m2

 MOCNESS equipped 

with nine 64μm mesh nets opened at 10m depth intervals. Both net systems had environmental 

sensing packages which recorded depth, temperature, and conductivity at 4 second intervals. The 

1-m2

 MOCNESS additionally recorded fluorescence and down-welling light intensity in 1995 - 

1999. Larval fish shrinkage is known to vary widely with water temperature and the time 
4 

between death and preservation (Theilacker 1980; Hay 1981; Lindner 1996). To minimize 

shrinkage, plankton samples were processed as quickly as possible and preserved in a 5% 

formaldehyde and seawater solution. 

 Ichthyoplankton were sorted from the 1-m2

 MOCNESS samples, identified to species, 

measured to the nearest 0.01mm, and transferred to 85% Ethanol. A gadid subsample was taken 

from each net to be used for gut content analysis (Table 1). Each subsample consisted of up to 

ten randomly selected cod and haddock larvae in each of 4 size categories: 3-5mm, 6-8mm, 9-

13mm and 14+ mm. Each larva was weighed to the nearest 0.001g by using a Mettler AC 100 

electronic balance. Morphometric measurements to the nearest 0.01mm were taken using a 

dissecting microscope with video camera and Optimas image analysis software. Wet weight, 

standard length (SL), body height, maxillary length, and skull width were recorded. All larval 

measurements were corrected for shrinkage by using Theilacker’s (1980) algorithm. With the aid 

of a dissecting microscope, the entire digestive track was removed from each larva and opened 

with mounted surgical needles and forceps. Gadid larvae do not have a differentiated stomach 

until ~15mm SL, (Economou et al. 1991) so prey items were removed and analyzed from the 

entire digestive track. Each prey item was identified to the lowest taxonomic and life stage 

possible. Prey item length and width measurements were made to the nearest 0.02mm with an 

ocular micrometer. Adult copepod and copepodite measurements included only the 

cephalothorax. It was assumed there was no shrinkage of prey items from digestion or 

preservation. Average measurements for each prey type from all larvae of the same year, species, 

and size category were used to approximate prey measurements which could not be determined 

because of advanced digestion or fragmentation. Prey biomass was estimated by using the length 

to dry weight conversion equations from Cohen and Lough (1981) for prey under 1.5mm in 

length and literature values (Davis 1984) for larger prey. 

 A Folsom plankton splitter was used to take a zooplankton subsample of 500-1000 

individuals from the 1⁄4-m2

 MOCNESS samples. All individuals in the subsample were identified 

to species and life stage. Plankton counts were averaged from all depths at each site and were 

standardized to number per cubic meter. Lengths were assigned from literature values (Murphy 

and Cohen 1978; Davis 1984), and biomass was estimated with the same methods used on the 

prey items. Larval fish predators and zooplankton over 2.5mm in length were considered too 

large to be selected as prey items and were not included in the potential prey field. 

 Prey was grouped into 28 categories based on species and life stage. Pseudocalanus spp. 

and Oithona spp. eggs, which are carried in sacs by the female, were included in biomass 

calculations for the adult stage but were counted as a single prey item with the adult female. 

Because of thier low occurrence as prey items in all years, Centropage spp., Temora longicornis,

and Metridia lucens were combined into the Copepoda category. Individually, these species were 

too rare to be statistically relevant. Combining them into the Copepoda category allowed these 

data to be included in the Chesson's calculations and statistical analysis. 

The Index of Relative Importance (IRI, Pinkas et al. 1971) was calculated for all fish 

containing prey. IRI was selected as it accounts for prey count and biomass as well as the 

occurrence of the prey item in the population. 

 IRI =%O  %N  %W 

 %O = percent occurrence of the prey item in all larvae of the same sampling category 

 %N = the proportion of the prey item in ingested prey count 

 %W = the proportion of the prey item in ingested prey biomass. 

IRI values are presented as %IRI to facilitate comparisons (Cortés 1997). 
5 

%IRI = 



n

i

i

i

IRI

IRI

1

n=the number of prey categories 

 Larval prey selectivity was estimated by using Chesson’s selectivity index, α (Chesson 

1983). 

ri = percent count of prey type i in larval diet 

ni = percent count (mean number m3

) of prey type i in prey field 

m = total number of prey categories, 1/m is neutral selection 

Chesson's α value is a commonly used selectivity index allowing easy comparisons with other 

larval feeding studies. Phytoplankton, Other, Calanus finmarchicus egg, and Sand prey 

categories were not utilized to classify selectivity because they were not quantified in the prey 

fields. Chesson (1983) states that when a food type is rare, it provides a poor opportunity for 

estimating preference, and thus, there can not be much confidence in the α value. For this reason, 

α values were not calculated when a prey category represented less than 0.5% of the total prey 

field. 

A modification of Colwell and Futuyama’s (1971) estimation of niche overlap (PS) was 

used to show predator and annual similarities in prey selection. 

1 0.5 (% % 2)

1

PS IRI 1 IRI

m

   

m = total number of prey categories 

%IRI1 = value of the first species or site being compared 

%IRI2 = value of the second species or site being compared 

PS varies from 0 (indicating diets are entirely different) to 1 (indicating diets are identical). 

Interannual comparisons by site were virtual estimations of niche overlap. The larvae being 

compared were not competing in the same space or time for the food resource. Annual niche 

overlap values between cod and haddock larvae were actual comparisons with the larvae 

competing in the same space and time for the resource. 

 Prey count, mean prey length, and total stomach biomass of individual larvae were fitted 

with a first order regression to show relationships with larval SL. Breaking the annual data down 

by larval size, site, and depth or time of capture created data groupings too small to be 

statistically significant, so larvae from all years were combined to analyze the effects of depth 

and time of capture. Annual depth-stratified prey fields and larval stomach content data for the 

mixed and stratified sites are presented in tabular form in the appendix. 

RESULTS 

 Water column thermocline depth and strength, water temperatures, and salinity values 

(Fig. 2) varied annually between sites. Bottom temperatures during the study period ranged from 

5.5 to 9.0oC, surface temperatures varied from 6.5 to 11.0oC, and salinities ranged from 32.0 to 



 m

i

i

i i

r ni

r n

1

( / )

( / ) 
6 

33.5psu. The stratified sites in 1994 and 1995 exhibited the widest temperature and salinity 

ranges. The stratified site had a gradual thermocline from the surface to 30m in 1993, 1994, and 

1997. A stronger thermocline from the surface to 15m was present in 1995 and 1999. The mixed 

site had generally uniform water column temperatures, except in 1997 when a thermocline was 

present from the surface to 30m. The highest salinities were found in 1994 and in the bottom 

half of the water column in 1995. The surface waters of the mixed site in 1997 and the stratified 

sites in 1997 and 1999 had the lowest salinities. 

 The zooplankton prey field in all years (Fig. 3) was dominated by 3 species of copepods: 

Calanus finmarchicus, Pseudocalanus spp., and Oithona spp. The Copepoda category combined 

all other copepod species collected and included Centropages typicus, Centropages hamatus, 

Paracalanus spp., Metridia lucens, Microsetella spp., and Temora Longicornis. In 1993-1994 

the main species in the Copepoda category were Centropages spp. and Temora longicornis. 

1995 the Copepoda category had about equal numbers of Centropages spp., Temora longicornis, 

and Metridia lucens. The 1997 and 1999 Copepoda category dominant species were Temora 

longicornis and Metridia lucens. Combining all available zooplankton prey species gave mean 

prey density of 6366 m-3 during the study period with a mean prey length of 0.50mm. Total 

available prey densities varied from 3413 m-3 at the 1994 mixed site to 9706 m-3 at the 1995 

stratified site. The stratified site had slightly higher prey densities than the mixed site for all 

years. Prey density also varied positively with increasing water temperature. The mean length of 

the zooplankton increased with increasing site temperature. Mean lengths ranged from 0.36mm 

at the 1993 mixed site to 0.70mm at the 1999 stratified site. Pseudocalanus spp. was numerically 

dominant in 1993 and 1994. The higher mean prey densities from 1995-1999 were caused by an 

increase in C. finmarchicus and Oithona spp. while Pseudocalanus spp. numbers declined only 

slightly. 

 Larvae in the 3-5mm size class of both species fed predominantly on Pseudocalanus spp. 

nauplii (Fig. 4). In 1993 mixed site larvae of both species were also consuming adult 

Pseudocalanus spp. In 1994 only 3-5mm cod larvae were consuming adults. Pseudocalanus spp. 

adults in 1994 were larger, averaging 1.1mm in length compared to 0.8mm in length in 1993. 

The larval diets in 1995, 1997, and 1999 did not include many adult Pseudocalanus spp. but 

were more diversified. They included smaller Oithona spp. nauplii and Calanus finmarchicus

eggs in addition to Pseudocalanus spp. nauplii. In 1999 3-5mm haddock also consumed 

phytoplankton of the genus Peridinium and Ceratium. No cod larvae in the 3-5mm size class 

were caught at the mixed site in 1999.

 Larvae in the 6-8mm size class preyed on more diverse prey ranging from naupliar to 

adult copepod life stages (Fig. 5). At the mixed site in 1993 and 1994 larvae of both species fed 

almost exclusively on gravid female Pseudocalanus spp. Larvae at the stratified site also 

consumed gravid Pseudocalanus spp. but continued to prey on Pseudocalanus spp naupliar 

stages. Larvae in 1995 and 1999 showed no strong prey preference at either sampling site. They 

preyed on all life stages of Oithona spp., Pseudocalanus spp., and C. finmarchicus eggs. In 1997 

larvae at both sites consumed only the naupliar and early copepodite stages of Pseudocalanus 

spp. The adult Pseudocalanus spp. and late stage copepodites present in the 6-8mm larval diet in 

other years of the study were available but not selected in 1997. 

 The diet of 9-13mm larvae consisted of larger copepodite and adult life stages (Fig. 6). In 

1993 and 1994 larvae continued to consume gravid female Pseudocalanus spp. At the stratified 

site in 1994, larvae also preyed upon Oithona spp. adults. Larvae from 1995 and 1999 had a 

more varied diet consisting of all the copepodite stages of Oithona spp. and Pseudocalanus spp. 
7 

In 1995 at the mixed site, larger cod larvae also preyed on the early copepodite stages of C. 

finmarchicus. No larvae from the 9-13mm size category were caught in 1997. 

Chesson’s α values showed positive selection shifted from nauplii to copepodites and 

adult copepods with increasing larval size (Table 2). Recently-hatched 3-5 mm larvae 

preferentially preyed on the naupliar stages of Pseudocalanus spp. and Oithona spp. in all years. 

In 1995-1999 3-5mm larvae had α values indicating negative selection of the adult and 

copepodite stages of these two species. Positive selection of Pseudocalanus spp. for 6-8 mm 

larvae varied between years and sites. In 1993, 1994, and 1999 there was strong positive 

selection for adults with weaker positive selection for naupliar stages. In 1995 and 1997 

Pseudocalanus spp. nauplii were still strongly preferred, with lower but still positive values for 

copepodites and adults. 6-8mm larvae from all years had positive selection for Oithona spp. 

adults. Chesson’s α values for larvae from the 9-13mm size class indicated a shift to positive 

selection for larger prey. Larvae had positive selection for adults and late stage copepodites but 

mostly negative selection of earlier life stages of both Pseudocalanus spp. and Oithona spp. 

Because of low occurrences as prey, Chesson’s α values could not be consistently calculated for 

C. finmarchicus and the Copepoda categories. The α values available for Calanus finmarchicus

indicated negative selection by larvae of all size classes in all years except 1995. 1995 cod in the 

9-13mm size class had positive α values for C. finmarchicus stage I - IV copepodites. Chesson's 

α values in the Copepoda category represent predation predominantly on Centropages spp. in 

1993-1994, shifting to mostly Temora longicornis in 1995-1999. 

Trends in individual feeding were similar for cod and haddock (Figs. 7-8). There was a 

positive correlation between increasing larval SL and higher prey count, stomach biomass, and 

mean prey length. These correlations were stronger for haddock in all years of the study. Total 

prey counts varied widely and had the weakest correlations (cod x r = 0.37, haddock x r = 0.45) 

and the shallowest slopes relative to larval size. Mean prey length consumed by individual larvae 

showed stronger positive correlations with increasing SL (cod x r = 0.49, haddock x r = 0.57). 

In 1993 and 1994, mean prey length had a bimodal distribution caused by larvae of the same size 

consuming both copepod nauplii and female Pseudocalanus spp., but not the intermediate sized 

copepodites. Mean prey size in 1995 and 1999 increased with larval length more gradually than 

in 1993-1994, and the relationship in 1997 had the lowest slope, indicating very little increase in 

prey size with increasing larval SL. All years showed a strong positive correlation between total 

biomass consumed and increasing larval SL (cod x r = 0.59, haddock x r = 0.62). Lower 

stomach biomasses in 1997 were caused by larvae consuming equivalent prey numbers but 

selecting smaller sized prey compared to other years of the study. 

 Interannual niche overlap values show that 1993 and 1994 had the most consistent high 

overlap values for both gadid species (Table 3). No other interannual patterns were readily 

apparent. Niche overlap values between cod and haddock larvae of the same year and size class 

were consistently high (Table 4), with a mean PS value of 0.72 and 89% of PS values over 0.5. 

There was no significant difference between the annual PS values for cod and haddock within 

each size class (Mann-Whitney Rank Sum Test). The lowest overlap was in the 3-5 mm 

categories for 1993 and 1994. These lower values reflect cod larvae feeding on Pseudocalanus

spp. adults at a smaller size than haddock. PS values comparing the mixed and stratified site 

larvae showed considerable high overlap in all years (Table 5). Location in a well mixed or 

stratified environment did not seem to strongly affect the species and life stage of prey consumed 

by early gadid larvae. 
8 

 A Mann Whitney Rank Sum Test run on all the larvae from this study showed prey 

count, total stomach biomass, and mean prey length consumed by individual larvae, varied 

significantly between larvae taken from the mixed and stratified sites (Table 6A). A series of 

Kruskal- Wallis ANOVA on Ranks tests was run to see if the differences seen between the 

sampling sites were influenced by depth. Larvae from the mixed site, which had consistent 

oceanographic conditions with depth, had little significant diet variance with depth (Table 6B). 

At the stratified site, which had varying water column oceanographic conditions, all but the 3-

5mm larvae showed significant dietary variance with depth (Table 6C). 

 Since mixed site larvae did not show significant differences in diet with depth, only the 

stratified site prey data were analyzed by 10m depth intervals (Fig. 9). Mean prey counts for both 

cod and haddock larvae show an increase at 20-30m depth, the average depth of the thermocline 

during the study years. Mean prey length increased with depth for all larvae. Total stomach 

biomass had two areas with higher stomach biomasses: near the thermocline depth because of 

increased numbers of prey consumed and near the bottom because of the larger size of prey 

consumed. 

 A second series of Kruskal-Wallis ANOVA on Ranks tests that compared feeding of 

larvae grouped in one hour bins determined by time of capture showed prey counts, mean prey 

size, and total stomach biomass were all significantly different. Mean prey size, mean prey 

count, and mean total stomach biomass for all larvae captured within a one hour increment were 

calculated and fitted with a third order regression. There was no discernable time pattern to mean 

prey size ingested in hourly increments; however, both cod and haddock larvae showed a strong 

diel pattern for prey count and mean stomach biomass (Fig. 10). Correlation with the fitted 

curves was good, with r values ranging from 0.43 to 0.75 and a mean of 0.65. All larvae had the 

lowest prey counts and stomach biomass between 0400 and 0800 and highest counts and prey 

biomass around 2000 in the evening. Cod larvae showed less variation in prey counts and 

stomach biomass with time than did haddock. 

DISCUSSION 

The broad trends in larval feeding from this GLOBEC study matched earlier field studies. 

Feeding incidence was over 95% for all size classes but was lowest for the 3-5 mm size class. 

Early larvae with no food in their gut often had yolk sac remnants, indicating they may not yet 

have begun feeding. It is possible these early larvae were eating protozoa (de Figueiredo et al. 

2005), which digest too quickly to be detected by this study's sampling methods. Protozoa were 

identified in pump sampling done concurrently with the MOCNESS sampling and so were 

available to the larvae as possible prey. Field studies covered in Heath and Lough's (2007) 

synthesis of literature describing larval cod diets showed that Pseudocalanus spp. and

Paracalanus sp. were the dominant species of prey for early cod larvae found south of 55' 00"N 

while Calanus spp. dominated in the northern part of the range. Chesson's α values showed both 

cod and haddock larvae from the southern flank of Georges Bank had strong positive selection 

for all life stages of Pseudocalanus spp. Studies from the Scotian Shelf indicated larval gadids 

preyed on Pseudocalanus spp. but also preyed preferentially on Paracalanus sp. (McLaren and 

Avendaño 1995; McLaren et al. 1997). Pseudocalanus spp. and Oithona spp. were the most 

numerous zooplankton species on the southern flank of Georges Bank in May 1993-1999. 

Paracalanus sp. had low mean abundances in May of 1993-1999, ranging from 320 m-3 to less 

than 10 m-3, so were not readily available as prey to larvae on Georges Bank. Larvae on Georges 
9 

Bank showed positive selection for most stages of Oithona spp. rather than selecting 

Paracalanus sp. like Scotian Shelf larvae. The Georges Bank larvae in this study appear to have 

substituted the available prey item, Oithona spp., which is the same size range as Paracalanus

spp. This changeover suggests that prey size and availability may be more important than prey 

species in determining positive selection. 

The lack of variability in larval gadid prey species selection during this 5 year study is 

mirrored by prey fields that did not vary extensively. Cluster and nonmetric multidimensional 

scaling (MDS) analysis of the National Marine Fisheries Service MARMAP and Ecosystem 

Monitoring plankton survey data showed the community structure of the zooplankton during the 

five years of the study period was strongly grouped (Kane 2007). The same type of analysis, 

using the GLOBEC broadscale survey copepod data from May 1995-1999, indicated the 

southern flank stations in the tidal front region of the study were closely clustered with high 

abundances of Pseudocalanus spp. and Oithona spp. (Durbin and Casas 2006). The slight 

tendency from 1993 to 1999 of increasing prey species diversity in the larger two larval size 

classes parallels an increasing trend in the mean Shannon diversity index (Kane 2007). 

The diets of cod and haddock larvae from individual years were comparable with high 

niche overlap values. Niche overlap values between cod and haddock for all years of this study 

were highest for the 3-5mm size class ( x = 0.79) and lowest for the 9-13mm sized larvae ( x = 

0.66). Kane (1984) had similar dietary overlap values (0.77-0.88) between 3-7mm cod and 

haddock larvae compared in 1 mm size categories. Larval cod and haddock have comparable 

early life histories. Age at length through 45 days post hatch (Bolz and Lough 1988) and gape to 

larval SL (Rowlands et al. 2006) curves were not significantly different for the two species. This 

implies that cod and haddock larvae of the same size have similar ability to capture prey. Larval 

gadids consume their prey whole. This creates a defined prey size range whose lower end is prey 

too small to be detected and whose upper end is prey too large to pass through the larvae's mouth 

structure (Gill 2003). The smallest larvae fed almost exclusively on 0.2-0.3mm prey, which 

could explain Pseudocalanus spp. nauplii's consistent positive selection. All but stage VI 

Pseudocalanus spp. nauplii fall within the 0.2-0.3mm size range. Oithona spp. naupliar stages IIV are smaller and thus harder for the larvae to detect, and Calanus finmarchicus stage III-VI 

nauplii are larger than the larvae can easily ingest. Early larvae also consumed copepodites and 

copepod adults whose length suggests they would be too large to fit in the larvae's mouth 

structure. The larger copepod's cylindrical shape and 0.2-0.4mm cephalothorax widths require 

the same mouth gape as 0.2-0.4mm nauplii if they are sighted and ingested from an end rather 

than the side. Petric et al. (2009) noted differences in copepod escape behavior affected prey 

selection more than encounter rates. Oithona spp. and Pseudocalanus spp. have the slowest 

swimming speed of the available copepod species in the prey field, making them easier for the 

larvae to capture. Centropages spp., which was consistently not selected as prey, is an 

omnivorous copepod with more aggressive movements and higher swimming speeds (Davis 

1984). Prey species selection appears to be a combination of prey size and prey behavior rather 

than a true preference for a specific copepod species. 

There are some developmental differences between cod and haddock larvae which did 

affect feeding. Haddock larvae developed fin rays faster, had higher fin ray counts, larger fins, 

and a greater body depth than did cod larvae (Auditore et al. 1994). Kane (1984) noted that first 

feeding cod were more aggressive predators which fed on larger prey soon after yolk sac 

absorption, while haddock were more passive foragers relying on less mobile prey. The 3-5mm 

larvae in this study showed some similar behavior, but this was not consistent throughout the 
10 

study. Passive items such as Calanus finmarchicus eggs and phytoplankton were found in 

haddock diets in 1995-1999 but were absent in cod diets. In 1994, cod larvae shifted from 

feeding on copepod nauplii to adult copepod stages at a smaller size than haddock. 

Reiss et al. (2005) showed significant positive relationships between increasing larval SL 

and the number and biomass of the prey. In this GLOBEC Georges Bank study there was a 

strong positive relationship ( x r = 0.61) between increasing larval SL and stomach biomass but a 

much weaker positive correlation ( x r = 0.33) between SL and the number of prey in the 

stomach. The maximum number of prey consumed by individual larvae within a size class 

increased very little with larval size. This implies that for 3-13mm gadid larvae there was a 

maximum number of attack, capture, and consume cycles each larva could conduct in a day, 

regardless of prey concentrations or prey size. Reiss et al. (2005) examined cod larvae up to 

25mm in length. The inclusion of these larger, more developed predators would cause the 

relationship between larval SL and number of prey consumed to have a steeper slope and 

stronger positive relationships than those of the 3 -13mm larvae of this study. 

The mixed and stratified sites oceanographic and prey field differences between did not 

seem to be reflected in the high annual PS values ( x = 0.68) comparing the two sites. From 

1993-1999 no major annual differences in species selection were noted between the predation 

patterns of larvae taken from the mixed or stratified sites on Georges Bank. Larval cod from the 

well mixed crest water and stratified conditions on the Scotian Shelf also showed no differences 

in prey selection (Lochmann et al. 1997). Combining larvae from all years of the study allowed a 

statistical analysis between the two sites of prey count, prey biomass, and stomach biomass with 

depth. These diet variables did not vary significantly with depth at the mixed site where water 

column conditions were uniform. At the stratified site, where oceanographic conditions changed 

with depth, all three values showed significant variability with depth. Larvae from 20 - 30 m 

depth, which was the average depth of the thermocline on Georges Bank during the study, had 

higher stomach prey counts. Plankton prey are known to accumulate along fronts (Lough and 

Manning 2001; Lough et al. 2006) such as the thermocline present at the stratified site in all 

years of this study. In general, maximum feeding (prey/larva) occurs at intermediate levels of 

turbulence where prey density is greater than 10-20 prey l-1 (Lough and Mountain 1996). Mean 

water column prey densities for all years of the study were below this threshold, varying from 

9.7 prey l-1 in 1995 to 3.4 prey l-1 in 1994. However, binning the prey field in 10m intervals 

gives prey densities near the thermocline of 7.1 prey l-1 in 1993, 6.9 prey l-1 in 1994, 19.9 prey 

l-1 in 1995, 22.8 prey l-1 in 1997, and 18.6 prey l-1 in 1999. These increased prey densities 

coincide with the increased ingestion rates of larvae sampled near the thermocline. Mayer and 

Wahl (1997) predicted larvae may be more selective at higher prey densities. Larvae have the 

possibility of maximizing feeding gains by selecting larger or more nutritious prey (Gill 2003). 

This theory was not supported by the field data from this GLOBEC study where the mean 

biomass of prey items eaten by larvae sampled near the thermocline was not different from the 

mean biomass of prey items consumed by larvae at other depths. It is possible the higher prey 

densities caused by frontal aggregation at the study site were not high enough to cause the 

selective feeding noted by Mayer and Wahl's 1997 mesocosm study where stocked prey densities 

ranged from a low of 5 prey l-1 to a high of 300 prey l-1. 

Gadid larvae are visual feeders and are relatively inactive at night (Ellertsen et al. 1984). 

During this study, dawn was between 0430 and 0530, and dusk was from 1900-2000. Assuming 

the four hour gut evacuation rate used by Lough and Mountain (1996), the larvae in this study 

had falling ingestion rates starting just after sunset and increasing ingestion rates beginning about 
11 

an hour after dawn. Prey item biomass also varied hourly but with no pattern that could be 

statistically related to time of day. 

 The interannual variations seen in larval diets in this study do not seem to be correlated to 

RNA/DNA growth rates from Georges Bank gadids taken during the same time frame. Buckley 

and Durbin (2006) found a good correlation between early larval cod and haddock growth and 

prey biomass estimated for the GLOBEC years 1995-99. Larval growth, based on RNA/DNA 

ratio analysis, was relatively poor in spring of 1995 when concentrations of their principle prey, 

Pseudocalanus spp., were lowest and greater in 1997 and 1999 when Pseudocalanus spp. 

biomass was higher. Several individual based models (IBM) use the total biomass ingested 

calculated using this study's database as the feeding variable used to calculate growth 

(Kristiansen et al. 2009; Lough et al. 2005, 2006). Looking at the slope of the regressions 

correlating total stomach biomass to larval SL shows 1995 and 1999 have the largest stomach 

biomass increases with larval size, followed by 1993 and 1994 with 1997 having the smallest 

biomass increase. This pattern suggests the highest growth rates from this study should be 1995 

and 1999 with the lowest in 1997. No larvae from the largest size class were caught in 1997, so it 

is possible the slope of the regressions could be falsely depressed. Excluding the 9-13 mm size 

class from the other 4 years of the study decreased the regression slope values, but 1997 still 

remained the lowest. 

Interannual differences were apparent in this study, but general trends were similar to 

previous field-based feeding studies. This study confirmed that feeding observations from shorter 

time scale studies were not one-time patterns but were generally consistent over the longer, 7 

year time frame of this study. While oceanographic conditions, larval size, larval capture time, 

depth, and capture site all contributed to observed feeding patterns, prey size, movement, and 

availability seem to have the strongest influence on prey selection. Continued use of this 

database to incorporate field-based depth-stratified and time-based data sets for modeling will 

help further the understanding of how each of these variables interact with oceanographic 

conditions to create the feeding patterns, growth curves, and mortality rates seen in this and other 

field studies. 

ACKNOWLEDGEMENTS 

 We wish to thank the crews of all the research vessels that helped collect these samples. 

This is contribution # 681 of the NW Atlantic US GLOBEC program jointly funded by NOAA 

and NSF. 
12 

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15 

Table 1. Number of larval stomachs processed in each size category. COD = Gadus morhua

HADDOCK = Melanogrammus aeglefinus 

CRUISE YEAR SITE COD HADDOCK 

ZOOPLANKTON 

 3-5 6-8 9-13 3-5 6-8 9-13 mean 

ct 

mean 

size 

AL9306 1993 Mixed 8 189 57 98 99 31 6672 0.36 

 Stratified 32 59 64 148 277 50 4530 0.37 

AL9403 1994 Mixed 19 28 9 97 99 31 3414 0.43 

 Stratified 30 41 7 148 276 50 4847 0.41 

SJ9507 1995 Mixed 8 22 17 16 32 9 7138 0.44 

 Stratified 31 72 22 11 111 50 9706 0.40 

OC9706 1997 Mixed 13 29 0 65 48 1 4815 0.62 

 Stratified 9 16 0 113 38 0 8942 0.64 

EL9905 1999 Mixed 2 90 36 57 128 51 6415 0.53 

 Stratified 5 86 79 82 213 157 7189 0.70 

TOTAL 157 632 291 835 1321 430 
16 

Table 2. Yearly Chesson's α values for each prey category for mixed site cod (cm), stratified site cod (cs), mixed sstratified site haddock (hs). 4.8 is neutral selection. Gray shading indicates positive selection. cod = Gadus morhu

Melanogrammus aeglefinus. 

3-5mm 1993 1994 1995 1997 

cm cs hm hs cm cs hm hs cm cs hm hs cm cs hm 

Calanus n1-3 5.1 Calanus n4-6 Calanus c1-2 Calanus c3-4 Calanus c5 Calanus adult Pseudo n1-3 4.4 40.0 7.1 20.2 26.4 14.6 65.2 40.8 36.3 35.2 21.2 33.5 49.8 42.3 51.8 Pseudo n4-6 1.0 2.5 44.3 13.5 12.8 24.4 25.5 48.8 32.5 62.1 34.7 38.3 29.6 29.7 Pseudo c1-3 1.9 5.2 1.3 0.3 5.4 11.8 Pseudo c4-5 1.1 9.2 0.5 5.1 Pseudo adult 14.2 14.1 58.5 38.3 21.7 4.1 5.9 3.7 Oithona n1-3 30.5 7.9 1.3 3.4 3.2 19.9 4.1 Oithona n4-6 24.0 21.8 2.4 6.8 7.5 4.8 8.4 8.3 9.5 Oithona c1-3 3.7 1.0 1.8 1.6 3.3 Oithona c4-5 0.3 Oithona adult 42.7 1.3 2.6 3.1 1.4 2.3 4.8 Copepoda n1-3 6.2 3.9 3.5 6.9 1.2 1.9 0.8 Copepoda n4-6 3.0 5.6 15.6 3.5 Copepoda c1-3 Copepoda c4-5 Copepoda adult 
17 

Table 2, continued. Yearly Chesson's α values for each prey category for mixed site cod (cm), stratified site cod (c

haddock (hm) and stratified site haddock (hs). 4.8 is neutral selection. Gray shading indicates positive selection. chaddock = Melanogrammus aeglefinus. 

6-8mm 1993 1994 1995 1997 

cm cs hm hs cm cs hm hs cm cs hm hs cm cs hm 

Calanus n1-3 1.1 1.1 Calanus n4-6 3.5 Calanus c1-2 3.3 Calanus c3-4 1.7 2.7 Calanus c5 Calanus adult Pseudo n1-3 0.7 2.1 0.9 22.7 7.6 2.4 6.1 8.6 13.9 24.0 21.4 14.2 35.9 16.0 32.4 Pseudo n4-6 0.2 2.6 0.3 7.4 5.9 4.8 1.8 6.1 37.4 36.9 57.5 30.5 33.3 14.5 21.1 Pseudo c1-3 0.5 3.7 0.9 7.2 0.5 0.7 0.3 13.8 11.5 0.8 10.4 7.9 2.3 0.2 Pseudo c4-5 1.5 10.7 1.6 4.5 1.3 1.8 6.0 0.8 7.1 1.5 0.2 Pseudo adult 49.6 58.9 51.7 37.1 80.2 27.1 83.2 24.7 3.7 4.7 6.8 12.1 1.5 3.6 Oithona n1-3 2.2 12.2 0.6 1.3 1.1 1.0 1.6 3.0 0.9 Oithona n4-6 0.2 6.7 6.5 2.0 1.8 3.6 2.5 2.3 2.2 6.8 3.0 Oithona c1-3 1.9 0.6 0.6 0.1 1.9 2.3 1.8 2.2 4.6 0.5 1.4 Oithona c4-5 1.4 0.5 0.5 1.8 3.7 1.2 3.8 1.1 Oithona adult 9.9 29.9 42.4 8.9 7.2 5.8 7.1 6.9 23.2 Copepoda n1-3 0.1 1.4 0.7 2.0 11.5 0.2 0.8 0.2 Copepoda n4-6 1.0 6.3 1.1 4.2 3.5 2.1 1.0 0.8 Copepoda c1-3 0.4 Copepoda c4-5 10.5 4.7 Copepoda adult 22.8 3.1 
18 

Table 2, continued. Yearly Chesson's α values for each prey category for mixed site cod (cm), stratified site cod (c

haddock (hm) and stratified site haddock (hs). 4.8 is neutral selection. Gray shading indicates positive selection. chaddock = Melanogrammus aeglefinus. 

9-13mm 1993 1994 1995 1997 

cm cs hm hs cm cs hm hs cm cs hm hs cm cs hm 

9-13mm Calanus n1-3 0.2 Calanus n4-6 Calanus c1-2 3.1 7.9 4.5 Calanus c3-4 3.3 2.0 10.3 Calanus c5 Calanus adult Pseudo n1-3 1.4 0.6 0.5 0.7 2.0 0.9 5.5 1.7 Pseudo n4-6 0.1 1.4 3.2 1.7 0.3 0.4 11.1 8.7 11.8 8.8 Pseudo c1-3 0.5 4.8 0.5 4.5 0.5 9.3 28.1 17.2 Pseudo c4-5 3.2 14.9 0.6 2.8 3.4 0.6 0.2 8.8 32.5 15.6 17.5 Pseudo adult 94.6 55.0 93.3 72.2 96.6 11.5 96.7 26.0 26.9 14.2 25.3 27.9 Oithona n1-3 Oithona n4-6 0.4 0.4 Oithona c1-3 0.3 0.4 0.9 3.0 0.5 1.1 2.7 2.8 Oithona c4-5 1.1 0.5 3.2 3.0 16.1 4.6 Oithona adult 17.8 15.4 35.5 36.6 19.7 3.5 22.6 9.4 Copepoda n1-3 0.4 1.2 0.2 Copepoda n4-6 3.6 0.2 Copepoda c1-3 3.3 Copepoda c4-5 1.3 0.6 1.4 Copepoda adult 
19 

Table 3. Interannual niche overlap values (PS) for cod (Gadus morhua) and haddock 

(Melanogrammus aeglefinus) at the mixed and stratified site. Gary shading indicates niche overlap 

values greater than 0.50. 

by prey 

species 

3-5m 

cod 93 94 95 97 99 

3-5m 

had 93 94 95 97 99 

93 0.62 0.78 0.46 93 0.54 0.51 0.20 0.37 

94 0.56 0.55 94 0.67 0.40 0.59 

95 0.56 95 0.32 0.60 

97 97 0.62 

99 99 

 

3-5s 

cod 93 94 95 97 99 

3-5s 

had 93 94 95 97 99 

93 0.35 0.61 0.27 0.21 93 0.47 0.31 0.14 0.39 

94 0.38 0.66 0.52 94 0.47 0.65 0.90 

95 0.32 0.33 95 0.30 0.53 

97 0.83 97 0.65 

99 99 

 

6-8m 

cod 93 94 95 97 99 

6-8m 

had 93 94 95 97 99 

93 0.89 0.11 0.08 0.74 93 0.94 0.26 0.07 0.47 

94 0.14 0.15 0.69 94 0.31 0.09 0.48 

95 0.20 0.20 95 0.49 0.55 

97 0.12 97 0.28 

99 99 

 

6-8s 

cod 93 94 95 97 99 

6-8s 

had 93 94 95 97 99 

93 0.60 0.41 0.14 0.23 93 0.57 0.30 0.27 0.46 

94 0.30 0.17 0.30 94 0.39 0.27 0.62 

95 0.30 0.43 95 0.15 0.43 

97 0.11 97 0.27 

99 99 

 

9-13m 

cod 93 94 95 97 99 

9-13m 

had 93 94 95 97 99 

93 0.94 0.31 0.90 93 0.98 0.42 0.59 

94 0.26 0.85 94 0.43 0.60 

95 0.40 95 0.69 

97 97 

99 99 

 

9-13s 

cod 93 94 95 97 99 

9-13s 

had 93 94 95 97 99 

93 0.60 0.33 0.42 93 0.84 0.34 0.31 

94 0.19 0.53 94 0.47 0.43 

95 0.51 95 0.71 

97 97 

99 99 
20 

Table 4. Annual niche overlap values (PS) for cod (Gadus morhua) and haddock (Melanogrammus 

aeglefinus) larvae from each size class. Values account for both prey species and life stage. 

Cod vs Haddock 

 1993 1994 1995 1997 1999 mean 

3-5mm mixed 0.57 0.41 0.61 0.79 0.60 

3-5mm 

stratified 0.47 0.69 0.66 0.97 0.59 0.68 

6-8mm mixed 0.96 0.93 0.48 0.80 0.61 0.75 

6-8mm 

stratified 0.56 0.73 0.75 0.90 0.64 0.72 

9-13mm mixed 0.93 0.98 0.57 0.71 0.80 

9-13mm 

stratified 0.83 0.72 0.62 0.90 0.77 

mean 0.72 0.74 0.61 0.86 0.69 

Table 5. Yearly niche overlap values (PS) between larvae from each size category from the mixed 

and the stratified sites. cod = Gadus morhua had = Melanogrammus aeglefinus. 

 Mixed vs Stratified site 

 1993 1994 1995 1997 1999 mean 

3-5mm cod 0.66 0.88 0.65 0.77 0.74 

3-5mm had 0.43 0.76 0.57 0.97 0.79 0.70 

6-8mm cod 0.61 0.82 0.67 0.86 0.40 0.67 

6-8mm had 0.33 0.65 0.42 0.88 0.79 0.61 0.69 mean 

9-13mm cod 0.86 0.58 0.59 0.41 0.61 0.11 std 

9-13mm had 0.82 0.81 0.68 0.65 0.74 

mean 0.62 0.75 0.60 0.87 0.61 
21 

Table 6. The probability that the differences in the median values being compared are due to 

random sampling not larval capture site or 10 m depth binning. Probability <0.025 is significant 

and shaded gray. COD = Gadus morhua HAD = Melanogrammus aeglefinus. 

Mann- Whitney Rank Sum Test: probabliity <0.025 is significant 

Mixed vs Stratified Site 

 

prey count stomach 

biomass 

mean prey 

biomass 

3-5mm COD 0.581 0.014 0.004 

3-5mm HAD 0.005 0.013 0.204 

A 6-8mm COD <0.001 0.412 <0.001 

6-8mm HAD <0.001 <0.001 <0.001 

9-13mm COD <0.001 0.095 <0.001 

9-13mm HAD <0.001 0.006 <0.001 

Kruskal- Wallis Anova on ranks: probabliity <0.025 is significant 

Mixed vs Depth 

 

prey count stomach 

biomass 

mean prey 

biomass 

3-5mm COD 0.358 0.924 0.881 

3-5mm HAD 0.557 0.371 0.057 

B 6-8mm COD 0.128 0.158 0.287 

6-8mm HAD 0.669 0.006 0.005 

9-13mm COD 0.950 0.886 0.819 

9-13mm HAD 0.055 0.114 0.002 

Kruskal- Wallis Anova on ranks: probabliity <0.025 is significant 

Stratified vs Depth 

 

prey count stomach 

biomass 

mean prey 

biomass 

3-5mm COD 0.152 0.880 0.975 

3-5mm HAD 0.024 0.011 0.009 

C 6-8mm COD <0.001 0.006 0.021 

6-8mm HAD <0.001 <0.001 <0.001 

9-13mm COD <0.001 <0.001 0.717 

9-13mm HAD <0.001 <0.001 0.327 
22 

Figure 1. Map of the southern flank of Georges Bank showing the locations of the mixed (M) and 

stratified (S) sites in May for all years of the study. 
23 

Figure 2. Mean water column conditions at each site during the study. Values are based on 

oceanographic data from the 1/4m MOCNESS (multiple opening and closing net environmental 

sensing system) binned in 1m depth intervals. Horizontal bars represent the range of temperature 

in degrees Celsius (black) and salinity in practical salinity units (gray) present during the 

sampling period. Mean temperature is shown in bold black and mean salinity is shown in dark 

gray. 
24 

Figure 3. Yearly mean zooplankton prey field concentrations (number per m3) for the mixed and 

stratified sites based on the 1/4m MOCNESS (multiple opening and closing net environmental 

sensing system) samples. The prey fields depicted include only zooplankton in the size range that 

could be captured and ingested by 5-13mm SL gadid larvae. 
25 

Figure 4. Yearly percent index of relative importance (%IRI) for 3-5mm cod, Gadus morhua, (black) 

and haddock, Melanogrammus aeglefinus, (gray) for 28 prey categories. 
26 

Figure 5. Yearly percent index of relative importance (%IRI) for 6-8mm cod, Gadus morhua, (black) 

and haddock, Melanogrammus aeglefinus, (gray) for 28 prey categories. 
27 

Figure 6. Yearly percent index of relative importance (%IRI) for 9-13mm cod, Gadus morhua,

(black) and haddock, Melanogrammus aeglefinus, (gray) for 28 prey categories. 
28 

Figure 7. Yearly prey counts, mean prey lengths and total stomach biomass for cod (Gadus 

morhua) larvae. Each point represents the value for an individual larva. Plots are fitted with a 

simple regression showing the correlation (r) between increasing larval standard length (SL) and 

the three sets of data. 1993 prey counts had no meaningful correlation. 
29 

Figure 8. Yearly prey counts, mean prey lengths and total stomach biomass for haddock 

(Melanogrammus aeglefinus) larvae. Each point represents the value for an individual larva. Plots 

are fitted with a simple regression showing the correlation (r) between increasing larval standard 

length (SL) and the three types of data. 1994 prey counts had no meaningful correlation. 
30 

Figure 9. Mean prey count, mean stomach biomass and mean prey length for stratified site cod 

(Gadus morhua) and haddock (Melanogrammus aeglefinus) larvae sampled in 10m depth 

intervals. 3-5mm larval data is black circles (●), 6-8mm data is white squares (□) and 9-13mm data 

is gray triangles (▲). 
31 

Figure 10. Hourly mean prey count and mean stomach biomass for cod (Gadus morhua) and 

haddock (Melanogrammus aeglefinus). The 3-5mm larval data are depicted with black circles (●), 

6-8mm larval data with white squares (□) and 9-13mm larval data with gray triangles (▲). Values 

for each larval size class have been fitted with a 3rd order regression. 
32 

APPENDIX 

Prey field and larval feeding data for all years categorized by sampling site, sampling depth, 

larval size class, and larval species. 

Depths with no larvae and species categories without data have been removed from each table. 

Gray "TOTAL" columns represent depth integrated data for each sampling site and year 

Depth = mean depth of the net. Most nets sampled 10 m depth increments. Data with no depth 

 value indicated integrate the entire water column. 

Total # of nets = number of plankton samples processed in each data category 

Total # of fish = number of larvae sampled in each data category 

Count = Zooplankton mean number per m-3

 Prey mean number per individual larva 

Biomass = Zooplankton mean biomass (ug dry wt.) per m-3 

 Prey mean biomass (ug dry wt.) per individual larva 

N1-N3 = copepod nauplii stages 1-3 

N4-N6 = copepod nauplii stages 4-6 

C1-C3 = copepod copepodite stages 1-3 

C4-C5 = copepod copepodite stages 4-5 
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