Trident Seafoods, Inc. (“Trident”) appeals from a decision of the National Labor Relations Board (“the Board”) ordering the company to cease and desist from refusing to bargain with three unions that had been the collective bargaining units representing the employees of Trident’s predecessor employers for more than twenty years. An Administrative Law Judge (“ALJ”) found that two of the bargaining units were not appropriate and that Trident only had a successor obligation to bargain with the one appropriate unit under sections 8(a)(1) and (a)(5) of the National Labor Relations Act (“the Act” or “the NLRA”). The Board reversed the ALJ’s findings with regard to the purportedly “inappropriate” units and determined that Trident had a successor obligation to bargain with all three of the incumbent unions. We grant the petition to review with regard to one of the bargaining units but deny the petition and grant the Board’s cross-application to enforce the decision as it pertains to the other two units.
I. Background
Trident runs a seafood processing operation in the states of Washington and Alaska. In 1992, Trident purchased Farwest Fisheries, Inc. (“Farwest”), including the salmon canning facilities at North Naknek (“Nak-nek”) and Ketchikan, Alaska. After the purchase, Trident operated the two facilities in a manner substantially similar to its predecessor. Independent harvesters catch the salmon and transfer them to Trident’s tender boats, which convey the salmon to canning stations, where they are processed and then shipped to Seattle for distribution.
Farwest had collective bargaining contracts with (1) Alaska Fishermen’s Union, Seafarers International Union, AFL-CIO (“AFU”) covering tendermen, beach gang, and culinary employees at Naknek; (2) District Lodge 160, International Association of Machinists and Aerospace Workers, AFL-CIO (“IAM”) covering machinists and cannery operation mechanics at both plants; and (3) Inlandboatmen’s Union of the Pacific, Region 37, ILWU, AFL-CIO (“IBU”) covering “non-resident” processing employees at Ket-chikan who are hired in Seattle rather than in Alaska. All three unions had been the collective bargaining agents for employees of *113 Farwest and its predecessors for at least 20 years, but none of the three had been officially certified by the Board. In June-July 1992, each of the three unions requested recognition from Trident based on the fact that a majority of employees hired by Trident in each unit previously had been employed by its predecessor Farwest. Trident refused to recognize the unions.
For over 20 years, the AFU labor contract had been in effect with Farwest and its predecessors. AFU’s most recent agreement was a multiemployer contract lasting from 1991 to 1994, and covering beach gang, culinary, and tendermen employees at the Nak-nek facility. When Trident succeeded Far-west, 34 of the 64 beach gang, culinary and tendermen employees it hired at Naknek were from Farwest’s roster.
IAM, whose collective bargaining status vis-a-vis Farwest was also based on a mul-tiemployer contract, had been the exclusive bargaining agent of a comprehensively defined group of machinists, cannery mechanics, and related positions since at least 1970. Its most recent contract with Farwest was also to be effective from 1991 to 1994, but it covered both canning locations (Naknek and Ketchikan). When Trident succeeded Far-west, 12 of its 14 Naknek hires and 12 of its 13 Ketchikan hires in the machinist category had been employees of Farwest.
IBU represented a unit of nonresident processing employees (ie., those hired in Seattle out of the 48 contiguous states without domicile in Alaska) since at least 1970. 1 Processing employees prepared seafood at the Ket-chikan facility for cooking, freezing, canning, and packing. IBU’s most recent contract with Farwest, negotiated on Farwest’s behalf by Alaska Employers (a multiemployer group), covered the years 1989 to 1992. When Trident bought out Farwest, 27 of 46 “non-resident” processors that it hired had been Farwest employees.
In the proceedings below, the unions contended that the three historical units were appropriate and that Trident’s decision not to bargain with them violated sections 8(a)(1) and (a)(5) of the Act.
2
Trident conceded that there was indeed “substantial continuity” between Farwest and Trident so as to make Trident a “successor employer” within the meaning of
Fall River Dyeing Corp. v. NLRB,
After a hearing, the ALJ rejected Trident’s claim that the AFU was an inappropriate bargaining unit. The ALJ found sufficient similarities among the tendermen, beach gang, and culinary employees for their grouping to continue as an appropriate bargaining unit deserving recognition from Trident. In contrast, the ALJ found that the IAM and IBU units were no longer appropriate, the IAM because, although “[i]t [was] satisfactorily shown that the machinists at each facility perform work distinguishably different from other employees, and are so recognized in terms of wages and working conditions,” there were too many differences between the machinists at Naknek and those at Ketchikan for the evidence to “support[ ] a eommunity-of-interest showing.”
Trident Seafoods,
The Board upheld the ALJ’s conclusion that Trident had a successor obligation to bargain with the AFU, but reversed the ALJ’s findings as to the inappropriateness of the IBU and the IAM units. According tc the Board, the ALJ had used the improper measuring stick of whether the bargaining units would be found “appropriate” if the employees were being organized for the first time and so had not given adequate weight to the Board’s longstanding policy that a history of harmonious bargaining between a particular unit and an employer creates a presumption that the historical bargaining unit remains appropriate. Accordingly, the Board reversed the ALJ on the appropriateness of the IBU and the IAM units, and concluded that Trident had a successor obligation to bargain with all three established bargaining units. The Board ordered Trident to cease and desist from refusing to bargain with the three, unions and to post a notice to this effect at its facilities.
Trident petitions for review from the Board’s determination that Trident has a successor obligation to bargain with each of the three historical units. The Board filed a cross-application for enforcement of the same decision. This court has jurisdiction under 29 U.S.C. §§ 160(e) and (f) (1994).
II. Discussion
A reviewing court will overturn a decision of the Board only when it is unsupported by substantial evidence. We “may not ‘displace the Board’s choice between two fairly conflicting views, even though the court would justifiably have made a different choice had the matter been before it
de novo.’
”
NLRB v. Walton Manufacturing Co.,
Section 8(a)(5) of the NLRA makes it an unfair labor practice for an employer “to refuse to bargain collectively with the representatives of his employees.” 29 U.S.C. § 158(a)(5) (1994). A successor employer is required to recognize and negotiate with the bargaining agent of the predecessor’s employees if the bargaining unit remains appropriate and the successor does not have a good faith doubt of the union’s continuing majority support.
See, e.g., Fall River Dyeing Corp. v. NLRB,
No one contests the fact that Trident is a “successor employer” under the Act. 5 However, Trident challenges the Board’s conclu *115 sions that all three historical units remain appropriate and, additionally, that the AFU has majority status in its current unit. We address the latter issue first.
A. Majority Status of AFU
With regard to the AFU’s majority status, Trident argued to the ALJ for the first time in its post-hearing brief, after the record had closed, that twelve of the 64 individuals in the current unit are “supervisors” and thus should be excluded from the definition of “employee” under the Act. 6 Trident again made this argument in its exceptions before the Board. In the wake of the Board’s adverse finding on this issue, Trident renews its supervisor argument before this court, relying solely upon isolated testimony in the record from its Naknek plant superintendent about the role of the captains in the seafood operation. The record contains the following interchange between Trident’s counsel and the superintendent:
Q What is the captain’s responsibility?
A The captain’s responsibility is the safe operation of that vessel; setting work schedules; sailing times; deciding how much fish can be put on board or how much can’t be put on board; recommends hiring; and he does have the authority to fire crew.
Q So he is in charge of the crew?
A So he is in charge of the crew on the vessel.
Q Who assigns the work on the tender boat?
A The Captain.
* * *
Q Do captains attend management meetings?
A Yes, they do.
Q Can you give me an example on that.
A It’s preseasonal. [Our fleet attendant will] meet with the plant management and they’ll go over — we’ll go over current policies, estimated catches, where the fleet is going to be, the fishing fleet, where our processing fleet is going to be stationed, radio schedules, radio channels.
J.A. 215-16. Trident cited only one precedent in its post-hearing brief to support its supervisor argument.
7
In its brief before this court, Trident cites additional precedents for the proposition that “[t]he Board has frequently held that vessel captains ... are statutory supervisors.” Final Brief of Trident, at 12-13 (citing
Hydrolines, Inc.,
The ALJ’s two separate findings on the supervisor issue contradicted one another. In one part of his decision, the ALJ rejected
*116
Trident’s post-hearing supervisor argument on the merits. In the analysis section, he distinguished
Joint Employers
from the present case, explaining that “[tjhere' is no showing that the supervisory authority recited in that case for occupations of first mates, chief engineers, and pilots is at all present here,”
The Board rejected what it perceived to be the ALJ’s “inadvertent” statement that the tender captains
are
supervisors,
id.
at 739 n. 3 (J.A. 4 n.3), but appeared to affirm his contrary finding that supervisory authority had not been. shown in this case. After explaining that “ ‘[i]t is well established that the burden of proving supervisory status rests on the party asserting that such status exists,’ ” the Board stated: “[Trident] did not introduce any evidence to support [its] claim. We find, therefore, that tender captains are not supervisors” within the meaning of section 2(11) of the Act.
Id.
at 739 (quoting
Billows Electric Supply,
At oral argument on appeal, the Board disputed that Trident properly raised its supervisor argument before the ALJ. Upon reviewing the record, we agree with the Board that Trident waived its supervisor argument by failing to raise it until its post-hearing brief. It is a basic tenet of administrative law that each party to a formal adjudication must have a full and fair opportunity to litigate the issues to be decided by the agency. When one party utterly fails to raise a significant issue before the ALJ, the record developed with regard to that issue will usually be inadequate to support a substantive finding in its favor and, generally speaking, neither the ALJ nor the Board should consider such an issue. This principle is illustrated by cases such as
Conair Corp. v. NLRB,
where this court held that the Board may not reach the merits of an unfair labor practice charge unless notice adequate to provide a fair opportunity to defend on the issue occurs
before
the record is closed.
We apply similar principles here. In the present case, Trident never raised the issue of the supervisory status of the tender captains until it filed its post-hearing brief after the record was closed. Moreover, Trident has not produced evidence of any extenuating circumstances, such as previously unavailable evidence, that might excuse Trident’s failure to do so. At the hearing before the ALJ, only one witness (one of Trident’s plant superintendents) testified as to the duties and authority of tender captains, and this testimony was given as general background material. Because the General Counsel was not put on notice that the supervisor’ status of the captains was at issue, he did not have any real opportunity to cross-examine, the witness on this point or to provide eounterevidence.
In light of Trident’s unexcused failure to raise the supervisor issue in a timely manner, the ALJ and the Board should not have addressed it: As it was, the ALJ made two contradictory statements on the merits of the issue. Subsequently, at the Board level, instead of finding that Trident had waived any such claim, the Board dismissed the first ALJ statement as an “inadvertence,” and in effect upheld the second ALJ finding that the captains were not supervisors — but on the erroneous ground that Trident “did not introduce any evidence” to show they were supervisors and that, “therefore, ... tender captains are not supervisors and are properly included in the [bargaining] unit.” It was clearly an overstatement for the Board to assert that no evidence was introduced indicating that the captains were supervisors, but just as clearly there was every reason to reject the late-raised claim either for untimeliness or on the grounds that Trident had not met its burden of showing that the captains were supervisors due to the inadequately developed record on the issue. Had the issue been properly raised and the record developed further, the sole witness’s few remarks might have aided Trident in proving that the captains were supervisors under the Act, but under the circumstances, when the General Counsel did not believe that to be an issue, these remarks by themselves could not support a finding to that effect. Quite simply, the record was not adequately developed to support any finding on the merits, and the ALJ’s statement that “[t]here is
no showing
that the supervisory authority” described in
Joint Employers
“is at all present here” may be so read.
B. Appropriate Bargaining Units
Trident next argues that, for various reasons, none of the three bargaining units is “appropriate.” We reject Trident’s claims as to the inappropriateness of the AFU and the LAM and therefore uphold the Board’s findings with regard to those two units. However, we agree with Trident that the IBU is not an appropriate unit under the Act. We find that the historical status of the “non-resident” processors unit cannot outweigh the fact that the unit fails to conform reasonably well to Board standards of appropriateness, and we therefore reverse the Board’s finding that Trident has a successor obligation to bargain with the IBU.
Trident first contests its successor obligation with regard to the AFU, contending that the “smallest appropriate unit” would be a plant-wide unit consisting of all production and maintenance employees at Naknek, since there is no distinction among these workers “in terms of supervision, housing, meals, laundry, benefits, overtime policies or other working conditions.” Petitioner’s Brief, at 14. Similarly, Trident challenges its successor obligation to bargain with the IAM, arguing that the ALJ correctly decided that a single combined unit of both plants was not appropriate in light of “the sheer distance between the- two plants (over 750 air miles), the total absence of any machinists classification interchange between the two facilities, the separate supervision at the two plants,” as well as “the fact that the *118 canning season[s] themselves were not identical.” Id. at 22. 10 Thus, Trident claims that the Board erred in reversing the ALJ’s decision on these two unit issues.
We reject Trident’s claims with regard to the AFU and the IAM because we find that the Board’s conclusions that these two units remained appropriate are supported by substantial evidence. Trident’s claims rely on the flawed assumption that the Board was compelled to apply the traditional community-of-interest test for bargaining units,
11
or (in the ease of IAM) the presumption that plant-wide units are the most appropriate. Actually, the Board usually applies the community-of-interest and plant-wide unit tests only when delineating units of previously unrepresented employees, not, as here, when it is assessing historical units that have had long periods of successful collective bargaining.
12
In most cases, a historical unit will be found appropriate if the predecessor employer recognized it, even if the unit would not be appropriate under Board standards if it were being organized for the first time.
Indianapolis Mack Sales & Service,
In deciding whether established bargaining units remain appropriate, the Board “‘has long given substantial weight to prior bargaining history.’”
Marin Operating,
Trident has failed to
meet
its burden with regard to the AFU and IAM units. It makes no claim whatsoever that these two historical bargaining units were unworkable or that they failed to produce harmonious labor relations, so as to be repugnant to the Act. Moreover, the record as a whole demonstrates that the old unit lines conformed reasonably well to Board standards of appropriateness. The ALJ found that enough similarities among the tendermen, beach gang, and culinary employees existed to support a finding that the AFU was an appropriate unit,
13
and the Board affirmed this finding.
*119
As to the IAM, even the ALJ acknowledged that the machinists at the two geographically remote plants performed work and received benefits that were similar to each other and that distinguished them from other Trident employees.
Trident Seafoods,
Trident also challenges its successor obligation to bargain with the IBU, similarly claiming that the IBU is not an appropriate unit under the Act. Trident argues that the overwhelming similarities between nonresident and resident processing workers mean that “non-resident processors do not constitute a separate, identifiable group for the purposes of collective bargaining.” Petitioner’s Brief, at 18. Instead, Trident asserts, the smallest appropriate unit “would be com-. prised of either all processing,employees, or a plant-wide unit o[f] production and maintenance employees.”
Id.
at 20. In this instance, we agree with Trident that the IBU does not constitute an appropriate unit under the Act. Although the weight given to a prior history of collective bargaining is “substantial,” it is not “conclusive.”
A.C. Pavement Striping Co.,
introduce!] significant evidence that [the predecessor’s] units had been rendered obsolete by industry shifts or developments at [the' predecessor], and the Board had applied the presumption in favor of long-established units in disregard of this evidence, we would not hesitate to find the application of the presumption irrational.
.We find that this is a case in which the Board’s application of the presumption in favor of historical units is irrational. Indeed, the totality of similarities between resident and nonresident processors and their shared bargaining history make this an even stronger ease than
Indianapolis Mack
for finding that a formal historical bargaining unit fails to conform reasonably well to Board standards of appropriateness. In
Indianapolis Mack,
in which the Board accepted a remand from the Seventh Circuit and reversed its earlier determination of a successor bargaining obligation, the Board found that a single service-and-part bargaining unit was appropriate despite its historical, formal separation into two units because “the Union and the predecessor had treated the groups of employees as one unit for many years,” resulting in a “de facto merger” of the two units.
Moreover, neither the ALJ nor the Board
17
cited any evidence rebutting Trident’s evidence or showing that the limitation of the IBU to “non-resident” processors was reasonably consistent with Board standards of appropriateness.' The ALJ rejected “speculation” by the General Counsel about “the relative stability of the groups, their cultural incidents, and the greater likelihood that the nonresidents might have a higher rate of return or consecutive periods of employment,” finding that such speculation failed to counter the overwhelming similarities between the two groups.
For the foregoing reasons, we deny the petition to review as to the AFU and the *121 IAM units, but we grant it and reverse the Board’s finding as to the IBU unit.
So ordered.
Notes
. Another union represented resident processing employees hired locally in Alaska.
. Section 8(a)(1) of the Act provides that “(a) ... [i]t shall be an unfair labor practice for an employer — (1) to interfere with, restrain, or coerce employees in the exercise of the rights guaranteed in section 157 of this title.” 29 U.S.C. § 158(a)(1) (1994). Section 8(a)(5) makes it a violation of the Act for any employer “(5) to refuse to bargain collectively with the representatives of his employees, subject to the provisions of section 159(a) of this title.” 29 U.S.C. § 158(a)(5) (1994).
.The primary differences between the two plants cited by the ALJ included the geographic remoteness of the two plants, the lack of interchange between the two plants’ machinists, and their somewhat different worldng seasons.
. Although the Board and the ALJ disagree in their legal conclusions regarding the bargaining status of IAM and IBU, this disagreement does not of itself undermine the support for the Board's final decision. The issues in dispute here do not turn on credibility determinations or findings of fact, hut rather on the legal interpretations to be ascribed to those findings of fact. This court will usually defer to the Board with regard to legal interpretations of the NLRA.
. Under Board precedents, an employer is a successor if he purchases the assets and ongoing business of a predecessor, retains the predecessor’s employees in the same jobs, operates the same facilities, uses the same supervisors, and manufactures the same type of product.
See, e.g., Fall River Dyeing Corp. v. NLRB,
. Trident argued:
The Captain is a supervisor under the Act, because he possesses and exercises the authority to fire employees, and recommend hiring. Tr. 230. The Board has typically excluded first mates, engineers and pilots as supervisors in similar situations. Joint Employers, Port of Abbyville, La.,175 NLRB 84 (1969).
Respondent's Post-Hearing Brief, at 23.
. In that case,
Joint Employers at the Port of Abbeville,
.The Board also contends that, even if the captains were found, to be supervisors, Trident did not meet its burden of showing that the AFU lost its majority status because Trident failed to show how many of the twelve captains were previously employed by Farwest (such that they would be subtracted from the majority) and how many were not previously employed by Farwest (such that they would not be subtracted from the majority). Id. at 16-17.
. In its brief, the Board argues that, under existing precedents, a boat captain is not necessarily a supervisor and that closer analysis of the specific facts in each case is necessary to determine whether a captain exercises sufficient "independent judgment” to make him a supervisor. Brief for the NLRB, at 16 & n.4.
. Although acknowledging the long history of the unit, the ALJ nevertheless concluded that these factors added up to a "vivid lack of any appreciable community of interests between the two groups" of workers.
. Under the community-of-interest test, the Board evaluates unit appropriateness based on "the degree to which a group of employees share a ‘community of interests’ distinct from the interests of other employees of the company.”
Banknote Corp. v. NLRB,
whether, in relation to other employees, they have different methods of compensation, hours of work, benefits, supervision, training and skills; if their contact with other employees is infrequent; if their work functions are not integrated with those of other employees; and if they have historically been part of a distinct bargaining unit.
Id.
at 648 (citing
Kalamazoo Paper Box,
. In this case, both the AFU and the IAM had negotiated contracts for thése units with Trident’s predecessors for more than 20 years.
. The ALJ found that the tendermen and the beach gang employees were both somewhat dis
*119
connected from the rest of the shore operations, and that little evidence had been presented by Trident as to the characteristics of the culinary employees. The AU also noted that “the skills of machinists and basically unskilled nature of processor work distinguishes both of these classifications from the beach gang,” and that the wages, hours, and benefits of the tendermen and the beach gang were different from those of other plant employees.
Trident Seafoods,
. The "resident” processors consist of those processors hired in Ketchikan (as opposed to .those hired in Seattle). But "the vast majority of employees hired in Ketchikan each year at the cannery are not Ketchikan residents. Most come from the ‘lower 48’ for summer work.” Petitioner's Brief, at 17 (citing J.A. 236). Indeed, only about 30 of approximately 260 "resident” processors were actually Ketchikan residents. J.A. 236.
. Although the IBU unit is formally defined as including only nonresident processing workers, ■Trident points out that the record shows that, “for collective bargaining purposes, no distinctions were made for non-resident processing workers at Ketchikan.” Petitioner's Brief, at 17. Under its labor agreement, IBU was one of four listed unions who represented the processing workers under a combined contract and, under that agreement, there was no distinction made between resident and nonresident employees: "It is clear that all the wages, hours and working conditions under that labor agreement were the same for all processing employees, without regard to a particular union's representation." Id.
. The Board found that for many years,
collective bargaining was carried out in unison for both units and resulted in contracts which provided parts and service employees terms and conditions of employment which were identical in almost every respect except for job classifications and hourly wage structure.
Id.
.The Board merely stated that “[s]uch units have a long history in the industry” and that "[t]he record contains no evidence that would warrant finding that this long-recognized unit is repugnant to Board policy.”