A Striped Bass Forage Indicator for Maryland’s portion of Chesapeake Bay

Jim Uphoff

Maryland DNR, Fishing and Boating Services June 2018 Maryland’s 2014 Chesapeake Bay Agreement Forage Goal

By 2016, develop a strategy for assessing the forage base available as food for resident Striped Bass in MD’s portion of Chesapeake Bay Resident striped bass

Baltimore • After spawning, most DE Mostly males & some immature males females stay in MD mid- Bay (residents) MD

• Important fishery VA • Main year-round large predator Mostly females from spawning rivers to New England Maryland’s fisheries managers want practical guidance on….

(1) What forage is eaten? (2) Is there enough? (3) Can 1 & 2 be answered at low cost? MD striped bass indicator approach

• Forage availability + bass well-being. (How much forage and is it enough?)

• Use existing data and surveys

• Indicators linked biologically and statistically (not going into that today) – Not etched in stone – Can be modified and-or improved Bass forage Indicator – keep it simple, inexpensive, and (hopefully) meaningful

• Tractable for available staff

• Understandable to public, managers • (Human brain processes about 7 items without shutting down)

• Targets (good) and limits (bad) for indicators to judge status

• Summarize with a combined score Previous Bay diet studies indicate important prey

Age 0

Age 1

Ages 3+ Forage and striped bass metrics

Factor Metric

Resident bass Catch per recreational boat trip (Sept-Oct) abundance Potential Forage index / bass abundance index attack success Forage Proportion of bass in fall with empty guts availability Bass Proportion in fall without visible body fat Condition Bass Survival Age 3 index / juvenile index 3 years earlier Proportion with visible body fat (fish condition) is key metric • Other indicators’ targets and thresholds calibrated to body fat • Visible body fat index compared well to “gold standard” nutrition analysis (Jacobs et al. 2013) • Fat index indicates overall food intake & potential for starvation • Body fat responds fairly quickly (weeks) • Longest body fat time-series in fall (Fish Health Program) Condition: percent of Striped Bass (11 - 34 inches) without body fat during October-November (MD DNR’s Fish and Wildlife Health Program).

90% 80% 70% Threshold 60% 1998-2004 average 50% 40% 30%

Percent without fat without Percent Target = 1990 20% 10% 0% Long-term major prey trends, 1959-2017. Indices were standardized to common years (1 = 1989-2017 average). 16 Menhaden Seine 14 Anchovy Seine Spot Seine 12 Blue Crab dredge 10 8 6

4 Standardized Standardized index 2 0 Predator abundance: resident Striped Bass abundance index with target (best body fat) and threshold (poorest body fat) Catch per MD private boat trip, Sept – Oct.

4.5 MRIP index Threshold Target 4 3.5 3 2.5 2 1.5 1 0.5 Catch = number harvested & released

Catch Catch private per boat trip 0

1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 2007 2009 2011 2013 2015 2017 Potential attack success: standardized forage to Striped Bass ratios when body fat indices were available (1989-2017 mean = 1)

6 Menhaden S Anchovy S Spot S

5 Spot T Anchovy T Blue crab D

S = seine 4 T = trawl D = dredge 3

2 Standardized ratio Standardized 1

0 Use fall diet as indicator of prey availability

• Striped bass & forage well mixed • What did striped bass feed on? • How well did they feed? • CBEF diet study (2006-2013) • Fall diet from health sampling since 2014 • Other seasons require new effort ($$$$) Availability index: fall diet of “small” striped bass • 11 to 18 inches • Varied diet compared to larger fish • Important they survive to legal size (fishery based on this) • We don’t ignore “large” rockfish (> 18 in) • Size break necessary for analysis due to CBEF permit restriction (Jim Price), 2006- 2015 Grams prey consumed per gram of small (11-18 in) Striped Bass in fall hook-and-line samples. Age-0 forage dominates.

0.035 Anchovy Spot Blue crab White perch Striped bass Other 0.03 Menhaden 0.025

0.02

0.015

0.01

0.005 Grams Grams gram per forage Bass 0 Percent of small (<18 in) Striped Bass diet represented by major forage groups, by number, in fall.

100%

80%

60% Other Blue Crab Spot 40% Bay Anchovy Atlantic Menhaden 20%

0% Forage availability: percent of small Striped Bass (<18 in) with empty stomachs in fall . Spot and menhaden size is an important influence.

60% Body fat 50% threshold breached 40% Body fat target met

30% Overton

Hollis Griffin 20%

10%

0% Grams of prey consumed per gram of large (18-34 in) Striped Bass during October-November. Fall consumption dominated by YOY forage.

0.035

0.03

0.025

0.02 Menhaden 0.015

0.01

0.005

0 Grams Grams per forage Bass gram

Anchovy Spot Blue crab White perch Striped bass Other Natural mortality: relative survival of male Striped Bass to age-3 (spring gill net index / JI in yr-3). Y-axis scale is arbitrary. 140 Relative S Target Threshold 120

100

80 survival 60

40

Relative Relative 20

0 Status summary for each indicator

Score Description

1 At threshold

2 Near threshold

3 Avoid threshold

4 Approach target

5 At target Mean score summarizes all indices. Individual scores indicate uncertainty

Mean Rec CPUE Forage:bass 6 Body Fat Sublegal S P empty 5 Best

4

3 Score 2

1 Worst

0 1995 2000 2005 2010 2015 2020 Indicator Issues (1) • Forage availability ≠ relative abundance – Menhaden consumption varies more than JI – More abundant menhaden than index indicates and-or feeding efficiency changes? Sampling issue with menhaden JI or diet? – Small bass: Ratio of menhaden and spot size to bass size seems to influence feeding success (Small menhaden and spot easier to catch and handle) – Older bass may become more efficient at obtaining prey (behavior and learning?) Indicator Issues (2) • Low data contrast? – Low forage, bass vary

• Mix of linear, abrupt (threshold), and lagged condition and survival responses to forage

• Indicators sometimes contradictory

• Diet and condition not sampled over the year

• Fall diet may miss other episodes Indicator Issues (3) • Convenience vs designed sampling

• Statistical vs biological & management significance

• Working on including benthic invertebrate index (MD biomass density from BIBI) Management Thoughts 1 • Coastal assessments may miss regional issues • Natural mortality not constant as assumed (relative survival and ASMFC tag-based estimates for Bay’s legal fish) • Expected bass outcomes from low fishing mortality in Bay may not be realized – Escapement of young females – Yield from Bay fisheries Management Thoughts 2

– Benthic and pelagic prey in low regime – High rockfish population popular idea, but may not be good for Bay balance – Manipulating harvested forage for better recruitment may be ineffective because of weak influence of spawning stock – Harvest of more and-or smaller bass (if allowed and accepted) may not entirely balance prey – Worst conditions may be avoidable, best hard to meet Questions, comments?

Contact: [email protected] 443-258-6087