1. Spatiotemporal Comparisons of Density, Growth and Production of Young-of-the-Year Weakfish in Delaware Bay and Major Tidal Tributaries
Timothy E. Targett & Brian P. Boutin
University of Delaware
College of Marine and Earth Studies
Lewes, DE 19958

2. Weakfish (Cynoscion regalis)
Background
Estuaries serve as important seasonal nursery habitats for many ecologically and economically important species
Important to be able to determine/assess habitats in the ‘mosaic’ whose production per unit area is disproportionately greater than surrounding habitats
Higher densities, growth and survival of juveniles (YOY)
Habitats providing suitable physicochemical conditions, abundant prey and low predation risk that are thus important to fisheries (on both per unit area and total area bases)
Weakfish (Cynoscion regalis)

3. The Sustainable Fisheries Act of 1996 (Public Law 104-279)
Background
The Sustainable Fisheries Act of 1996 (Public Law )
Mandated the description, identification, and preservation of Essential Fish Habitat (EFH)
“Those waters or substrates necessary for spawning, breeding, feeding or growth to maturity”

4. Background
Four levels of information of increasing utility to designate EFH
Level 1: presence/absence
Level 2: habitat-specific densities
Level 3: habitat-specific growth, survival, and reproduction
Level 4: habitat-specific fish production
Data is often lacking for levels 3 and 4

5. NJ
The Delaware Estuary
Variety of available nursery habitats for young fishes
Tidal Tributaries
Open Bay
Spatial and temporal gradients of physicochemical and biological attributes
Temperature range of OC
Salinity range of 0-30‰
Variable DO and nutrient levels
Seasonal phytoplankton and vegetative marsh production
Seasonal fish production
Potential spatial patterns in growth and production as well DE

6. Weakfish (Cynoscion regalis)
Important seasonal component of fish assemblage
Ecologically and economically important species
Delaware State Fish!
Spawning activity in spring
Peak in late May and early June
Multiple cohorts (usually two)
Juveniles begin recruiting to nurseries in the early summer
Return to coastal ocean in the fall

7. Delaware Bay Three main studies on juvenile weakfish:
Paperno et al. 2000, Grecay and Targett 1996, Lankford and Targett 1994
Abundance highest in the upper estuary
Heavy recruitment in areas <20‰
Growth and survival:
Upper estuary
Lower growth rates, condition and gut fullness
[Higher survival]
Mid and Lower estuary
Higher growth rates, condition and gut fullness
Seasonal dynamics

8. Delaware Bay Tidal Tributaries
Two main studies have sampled juvenile weakfish
(NJ side)
Able et al. (2001) found highest abundances in upper estuary tributaries
Most abundant species in the upper estuary
Nemerson and Able (2004)
Upper and lower bay tributaries were similar in abundance in first year of sampling
Lower bay tributaries had higher abundances in the second year of sampling

9. Focus has been on the Bay proper NJ DE
Focus has been on the Bay proper
No studies have compared these two habitats in terms of habitat quality and nursery function for young weakfish
Opportunity to build upon what is known
Compare densities, growth and production
Using a new technique (RNA:DNA) to examine short-term growth rates and production

10. Objectives
Compare the spatial and temporal patterns in habitat-specific density, growth and production of YOY weakfish within and among six tidal tributaries of Delaware Bay and the adjacent open bay areas
Determine areas of highest density, growth and production of YOY weakfish and speculate on the most productive habitats in order to better define EFH for YOY weakfish

11. Methods Study Sites 6 Tidal Tributaries 4 Bay Stations
Blackbird Creek (B), Simons (S), St. Jones (SJ), Murderkill (MK), Mispillion (M) and Broadkill (BK) Rivers
Lower, middle and upper stations
~3km apart
4 Bay Stations
Outside the mouths of each river (except Blackbird Creek)
St. Jones and Murderkill Bay station is the same

12. NJ PA NJ DE DE Delaware Bay Blackbird Creek (B) Simons River (S)
St. Jones
River (SJ) DE
Delaware
Bay
Murderkill
River (MK)
Mispillion
River (M)
Broadkill
River (BK)

13. Methods Field Sampling Biweekly July through early October
Replicate 150 m tows
3 m otter trawl (38 mm stretch mesh net, 25 mm stretch mesh cod end, 5 mm square mesh cod end liner)
All sampling within 2 h of high tide
Temperature, salinity and DO measurements at each station
Up to 15 fish per site per day were frozen on dry ice for short-term growth rate (RNA:DNA) analysis

14. Methods YOY weakfish habitat-specific densities
Numerical densities calculated as number of weakfish caught per tow at a specific station on a specific day by the area swept (382.5 m2 for riverine stations, varied for Bay stations)
Biomass densities calculated as above using station-specific YOY weakfish mass per tow

15. Methods Weakfish in situ growth rate assessment
Requires a method that can estimate growth over very short time periods to afford small scale temporal and spatial comparisons
Essentially “instantaneous” growth rates
RNA:DNA / growth rate relationship

16. Methods No food Low ration Ad libitum Medium 20°C 23°C 27°C 30°C
RNA:DNA
Growth Rate
First step consists of calibrating RNA:DNA against known lab determined growth rates.
Carried out at Dunstaffnage Marine Laboratory and Port Erin Marine Laboratory.
Experiment replicated at each location using fish collected locally.
Fish held in individual containers. Growth rate, calculated from weight increments over a 10 day growth period. Fish were immediately frozen at end of experiment. RNA:DNA was measured in white muscle tissue and regressed against growth rate
Food type: Mysis relicta
N = 5 fish per treatment

17. Predictive Relationship
From Stierhoff, Targett & Power (submitted)

18. Temporal Sensitivity
RNA:DNA of juvenile weakfish responds in very short time periods (~48 h) to changes in feeding and growth conditions

19. HABITAT-SPECIFIC GROWTH AND PRODUCTION
OF YOY WEAKFISH
P= Gs*B
Gs is an individual’s short-term growth rate calculated from the RNA:DNA to growth rate relationship
B is the station-specific mean daily biomass density
 Snapshots of growth
 Snapshots of production

20. Results Multitude of Comparisons
Spatial
Bay, Tributary, Stations within Tributaries
Temporal
Sampling Weeks - Seasonality
Focusing on the major findings of the first year’s data (2005)

21. Seasonality Numerical Density Biomass Density Production Growth Rate
Seasonal decline in numerical and biomass densities
Seasonal decline in growth rate
Large drop in August
High temperature effects on SGR and food availability
Seasonal decline in production
Low growth in August and declining biomasses causing production levels to plummet
Slight rebound in September due to recovery of growth but production constrained by declining biomass

22. Overall Spatial Trends
Numerical Density
Biomass Density
Growth Rate
Production
Significantly higher numerical and biomass densities in adjacent Bay
Significantly higher growth rate in adjacent Bay
Some indication of higher production levels in adjacent Bay

23. Bay vs Tributaries
Numerical Density
Biomass Density
Tributary
Bay B
Growth Rate
Production BK
Numerical and biomass densities generally higher in the Bay than in the adjacent tributary locations
Growth rate consistently higher in Mispillion (M) and Simons (S) Bay locations
Production highest within the middle portions of the estuary
Lower Murderkill and Mispillion Rivers
Occasional coincident very high biomass and high growth rates in these rivers  high overall production values
 Suggests absence of density-dependent growth

24. Bay vs Tributaries
Numerical Density
Biomass Density
Tributary
Bay B
Growth Rate
Production BK
Production was also consistently high in the bay sites off the Mispillion and Simons Rivers, due to high biomass at both locations and high growth rates, particularly at the Mispillion Bay site

25. Conclusions To-Date
First year results suggest the most productive nursery areas for young weakfish in the Delaware Estuary are in the lower portions of mid-bay tidal tributaries and the adjacent open bay
Incidents of very high production within tributary habitats
Highest biomass densities frequently coincide with highest growth rates  high overall production values
No evidence of density-dependent growth
The approach used here has promise for examining patterns and processes underlying (facilitating or limiting) productivity at upper trophic levels in the Delaware Estuary

26. Acknowledgements National Estuarine Research Reserve Delaware NERR
Graduate Research Fellowship
Delaware NERR
Bob Scarborough, Mike Mensinger
T. Targett Lab
Graduate Students
Mike Rhode, Ben Ciotti, Damian Brady, Danielle Tuzzolino
Technicians
Scott Baker, Nate Campbell Catherine House, Taylor Unger